In the Kamen Rider universe, Mirror Monsters are man-eating monsters that exist within the Mirror World and much of the series focusses on their partnership with the Kamen Riders.
The Mirror Monsters are entities created by Yui Kanzaki as drawings made by her and her brother Shiro when they were young, given life by Yui's mirror-self. As these creatures have no true life of their own, they prey on human lifeforce to survive.
The only people they don't attack are Shiro and Yui, whom they obey absolutely. Any rogue Mirror Monsters who attack them are countered by Kamen Rider Odin or a loyal Mirror Monster who serves Shiro. However, the majority of the monsters are contracted allies and the power source for most of the thirteen Kamen Riders, building their power by absorbing the energy of other defeated Mirror Monsters.
When the energy of a defeated Mirror Monster isn't absorbed, it can form a new Mirror Monster.
Once a Mirror Monster is confined, it forms a Contract with the Rider who captured it and becomes labeled a Contract Monster (契約モンスター Keiyaku Monsutā?). However, the Contract Monsters can turn on their human partners and devour them if their Vent Deck is destroyed or should they decide to quit being Riders and thus starve the monster of the energy it would receive from absorbing the energies of other Monsters. Contract Monsters do have a sense of loyalty to their human partners (examples include Darkwing, Blancwing, Destwilder, and Magnugiga). At the same time, some will get angry at their Rider when forced to do something they choose not to do (Dragredder and Gigazelle are prime examples mainly due to the attitudes of their Riders). Then, there are those who completely show utter hatred for their Rider. In Ouja's case, after acquiring Metalgelas and Evildiver; his monsters constantly attack him. However, his unstable mentality and wild personality is more than enough to tame all three of his "pets." Beforehand, Metalgelas and Evildiver loyally served Gai and Raia respectively.
- Dragreder (ドラグレッダー Doragureddā?): A red mechanical Japanese dragon Contract Monster under Kamen Rider Ryuki, later gained enough power to evolve into Dragranzer (ドラグランザー Doraguranzā?). Attack by breathing fire.
- Darkwing (ダークウイング Dākuuingu?): A giant mechanical Vampire Bat-like Contract Monster under Kamen Rider Knight, later gained enough power to evolve into Darkraider (ダークレイダー Dākureidā?).
- Volcancer (ボルキャンサー Borukyansā?, 5-6) : A mechanical humanoid Crab-like Contract Monster under Kamen Rider Scissors. Fights using its large claws. It turned on its master and devoured Scissors when his Card Deck was destroyed. This monster was destroyed by Ryuki's Dragon Rider Kick with Darkwing absorbing the monster after Dragreder was told not to.
- Magnugiga (マグナギガ Magunagiga?, 10-50): A giant Ox-like robot (with a humanoid appearance similar to a minotaur) that is Kamen Rider Zolda's Contract Monster. Likely due to the size of the costume, it never fights on its own and is only summoned for Zolda's Final Venta. This monster was destroyed by Ouja's Doomsday in the series.
- Evildiver (エビルダイバー Ebirudaibā?, 13-50): A flying mechanical Stingray monster that was originally Kamen Rider Raia's Contract Monster. It attacks by ramming it enemies. It becomes a Contract Monster under Ouja when it attempted to avenge its master in vain.
- Metalgelas (メタルゲラス Metarugerasu?, 15-50): An armored humanoid Rhinoceros monster that was originally Kamen Rider Gai's Contract Monster. It becomes a Contract Monster under Ouja when it attempted to avenge its master in vain. It mostly fights by charging into enemies
- Venosnaker (ベノスネーカー Benosunēkā?, 19-50, 13-Riders): A giant mechanical Cobra monster that is Kamen Rider Ouja's Contract Monster. It fights either by biting or spitting acid. Is the largest Mirror Monster in the series.
- Destwilder (デストワイルダー Desutowairudā?, 35-47): An armored humanoid White Tiger monster that is Kamen Rider Tiger's Contract Monster. Survived its master and later destroyed by Ryuki Survive's Dragon Firestorm
- Gigazelle (ギガゼール Gigazēru?, 41-44): A humanoid Gazelle monster. The Gigazelles were originally rogue Mirror Monsters until they became Kamen Rider Imperer's Contracts.
- Goldphoenix (ゴルドフェニックス Gorudofenikkusu?, 50): A mechanical Phoenix monster that is Kamen Rider Odin's Contract Monster.
- Biogreeza (バイオグリーザ Baiogurīza?, 13 Riders): A humanoid Chameleon monster that is Kamen Rider Verde's Contract Monster.
- Blancwing (ブランウイング Buran'uingu?, Episode Final): A mechanical Swan monster that is Kamen Rider Femme's Contract Monster.
- Dragblacker (ドラグブラッカー Doraguburakkā?, Episode Final): A mechanical black Japanese Dragon monster that is Kamen Rider Ryuga's Contract Monster. It is a dark version of Dragredder. It was destroyed alongside its master.
- Psycorogue (サイコローグ Saikorōgu?, 39-43): A robotic humanoid Cricket that is Alternative Zero's Contract Monster. Survived its master upon being controlled by Yui, it was weakened by Ryuki's Drag Claw Fire to be destroyed by Knight's Hishou Zan
- Deadlemur (デッドリマー Deddorimā?, 13-14): A three-eyed Lemur/Tarsier sniper-type Monster that roamed around the elevators of Le Crock, starting the Non-Stop Elevator urban legend. Destroyed by Knight's Hishou Zan and absorbed by Dragredder. Deadlemur appeared in Shinji's dream during the Hyper Video special, among those destroyed by Zolda's End of World.
- Gelnewt (ゲルニュート Gerunyūto?, 17-18): A humanoid Newt monster that wielded a shuriken as its weapon. Destroyed by Ryuki's Dragon Rider Kick. Gelnewt appeared in Shinji's dream during the Hyper Video special, among those destroyed by Zolda's End of World.
- Sonorabuma (ソノラブーマ Sonorabūma?, 29, 38): A Cicada monster that targeted Shimada when Reiko and Shimada were abducted. Was kicked by Ryuki into the mirror and killed by Zolda's Giga Launcher and Knight's Hishou Zan. A second Sonorabuma appeared in episode 38, only to be destroyed by Knight's Hishou Zan.
- Brobajell (ブロバジェル Burobajeru?, 33, 43-44): A Jellyfish monster that used the electrodes on its forearms to kill its prey. Destroyed by Ryuki's Dragon Rider Kick. From the end of episode 43 to the beginning of episode 44, a Brobajell was among the Mirror Monsters that helped to protect Yui.
- Fake Kamen Rider Agito (偽仮面ライダーアギト Nise Kamen Raidā Agito?, Hyper Video): Labeled as the ruler of Mirror World, he is the mirror version of Kamen Rider Agito's Burning Form. He gathered the Mirror Monsters in a plan to destroy the Kamen Riders, which confused Ryuki to see the monsters actually use teamwork. But once Agito arrived, Ryuki found out his Dragon Rider Kick had no effect on the Fake Agito. Though it took Ryuki and Agito's Double Rider Kick to destroy him, Fake Agito was actually a dream and nothing more.
Though one type of Spider monster appeared in the show, three human-like types appeared in the 13 Riders special (though they made cameos in the show).
- Dispider (ディスパイダー Disupaidā?, 1-2): The first of the Mirror Monsters Shinji encountered, a giant gold and white Spider-like monster that targeted women like Ida Megumi. Though destroyed by Knight's Hishou Zan, Dispider was not absorbed by Darkwing and was reformed into a drider version of itself named "Dispider Reborn" (ディスパイダー・リ・ボーン Disupaidā Ri Bōn?), now attacking men as well. It was finally destroyed by Ryuki's Dragon Rider Kick and absorbed by Dragredder. In the 13 Riders special, a purple and brown-colored Dispider was the Mirror Man's final defense, devouring Gai before being destroyed by Knight Survive's Shippu Dan.
- Mispider (ミスパイダー Misupaidā?, 43-44, 13 Riders): A Purseweb spider monster. It appeared among the Mirror Monsters that helped to protect Yui. A Mispider appeared in the 13 Riders special, where it dragged Shinji into the Mirror World to eat until the first Kamen Rider Ryuki inferred. Despite receiving aid from Respider, the Mispider was destroyed by the previous Ryuki's Drag Claw Fire. Another appeared, attempting to eat a young girl, but it was killed by the Riders during the fight.
- Respider (レスパイダー Resupaidā?, 43-44, 13 Riders): It is a Jorō Spider monster. It appeared among the Mirror Monsters that helped to protect Yui. A Respider appeared in the 13 Riders special, where it came to Mispider's aid in fighting the previous Ryuki before he gave his Vent to Shinji. That Respider was destroyed by Ryuki's Drag Saber. Another appeared to aid another Mispider and a third Solospider, but both were killed by the Riders during each fight.
- Solospider (ソロスパイダー Sorosupaidā?, 43-44, 13 Riders): A green-armored Tetragnatha praedonia monster. A Solospider appeared in the 13 Riders special, where it fought Ryuki and Scissors until it was destroyed by Ryuki Survive's Meteor Bullet. A second appeared to fight the Riders when they got near to the Mirror Man.
Zelles are humanoid gazelle monsters that usually attack in groups, sneaky and blindingly fast with the ability to jump distances. Despite being rogue Mirror Monsters, they usually travel with Kamen Rider Imperer because of his contract to Gigazelle even appearing when he uses his Final Vent.
- Gigazelle: The soldier Zelles. A gold Gigazelle appeared in episode 3 to attack a girl and was destroyed by Ryuki's Drag Claw Fire. A green and yellow Gigazelle appeared and was destroyed by Ryuki's Dragon Rider Kick in the next episode. A third quickly appeared and was destroyed by Zolda's Giga Launcher without trouble. By episode 39, as a prelude to Impaler's arrival, a dozen Gigazelles were on the attack until they were all wiped out by Ryuki Survive's Dragon Firestorm. A Gigazelle appeared in Shinji's dream during the Hyper Video special. It was destroyed by Ouja's Veno Crash.
- Megazelle (メガゼール Megazeēru?, 3-4): A blue and yellow Zelle that devoured a school teacher before Ren could stop it, making it personal. Knight destroys the monster with his Hishou Zan, and it is absorbed by Darkwing.
- Negazelle (ネガゼール Negazēru?, 24): A Gazelle/Cape Buffalo-type Zelle that interfered in the fight between Ouja & Knight Survive. This monster was killed by Knight Surivive's Shippu Dan. Negazelle appeared in Shinji's dream during the Hyper Video special, among those destroyed by Zolda's End of World.
- Omegazelle (オメガゼール Omegazēru?, 24-25, 39): A Gazelle/Water Buffalo-type Zelle. It used a staff in battle. Targeted Shinji and Ren while they were unable to transform in episodes 24-25, before being destroyed by Ryuki's Dragon Rider Kick. Another appeared in episode 39 leading a pack of Gigazelles until it was destroyed by Ryuki Survive's Dragon Firestorm.
- Magazelle (マガゼール Magazēru?, 39, 44): A Bighorn Sheep/Gazelle-type Zelle. The first one encountered in episode 39 and destroyed by Tiger's Crystal Break. Another appeared in 44 was destroyed by Ryuki's Dragon Rider Kick.
Zebraskulls are zebra monsters able to stretch their bodies, using the blades on their forearms to slice and dice prey.
- Zebraskull Iron (ゼブラスカル・アイアン Zeburasukaru Aian?, 6-8): A white-headed Zebra monster that Zolda battled before it escape him. This monster ran loose until it fought Ryuki and was killed by his Dragon Rider Kick.
- Zebraskull Bronze (ゼブラスカル・ブロンズ Zeburasukaru Buronzu?, 7): A black-headed Zebra monster that is a servant to Zebraskull Iron. This monster supported Iron until it was destroyed by Zolda's Giga Launcher.
Boarders are wild boar-like Mirror Monsters that use their bodies as weapons in charge attacks.
- Wildboarder (ワイルドボーダー Wairudobōdā?, 9-10): A red Wild Boar monster that first appearing when Shimada was kidnapped by a man with a grudge against ORE Journal. Moving with incredible speed, Wildboarder grabbed the kidnapper and dragged him into the mirror world, unwittingly setting Shinji up as the crook with Knight attempting to kill the monster when Zolda interfered. When the monster later tries to eat the kidnapper's daughter Yukari, Ryuki and Knight fight it until Zolda takes advantage to destroy them all, with Wildboarder killed during the onslaught of Zolda's End of World.
- Shieldboarder (シールドボーダー Shīrudobōdā?): A blue Wild Boar monster. This monster is able to use its chest armor as a shield. This monster was destroyed by Ryuki's Dragon Rider Kick.
Biters are longhorn beetle-like monsters armed with boomerang sabers.
- Zenobiter (ゼノバイター Zenobaitā?, 11, 38, 43-44): A blue-colored male beetle monster. He feasted on people in a train car on the Fukutoshin Line. Though he keep Ryuki from getting to his weapons, the monster is tricked as he is then destroyed by Ryuki's Dragon Rider Kick. A second Zenobiter appeared in episode 38, only to be destroyed by Knight's Hishou Zan. From the end of episode 43 to the beginning of episode 44, a Zenobiter was among the Mirror Monsters that helped protect Yui. A Zenobiter appeared in Shinji's dream during the Hyper Video special, destroyed by Knight's Hishou Zan.
- Terabiter (テラバイター Terabaitā?, 11-12): A red-colored female Beetle monster. She pursued Ejima though held at bay by the Seal Card he had on his person. But once he loses the card, Terabiter almost devoured him when Shinji and Ren came to his aid in time. Destroyed by Ryuki's Dragon Rider Kick and Knight's Hishou Zan, absorbed by Darkwing. Terabiter appeared in Shinji's dream during the Hyper Battle Video special, among those destroyed by Zolda's End of World.
- Bakraken (バクラーケン Bakurāken?, 15): A blue squid monster with a light blue chest. It wields a tentacle-like whip in battle. It attacked the surviving member of the computer club. This monster was destroyed by Gai's Heavy Pressure.
- Wiskraken (ウィスクラーケン Wisukurāken?, 15-16, 38, 43-44): A blue squid monster with a red chest. It wields a staff in battle. It witnessed the death of Bakraken and later fought Kamen Rider Ryuki. It was destroyed by Ryuki's Dragon Rider Kick. A second Wiskraken appeared in episode 38, only to be destroyed by Knight's Hishou Zan.
Gulds are Kanzaki's bird-like Mirror Monsters, serving him in maintaining the Rider War by eliminating those who refuse to fight and those who get too noisy in his affairs. Guld means "Gold" in Danish.
- GuldThunder (ガルドサンダー Garudosandā?, 22): A red Fenghuang monster in a Phoenix-shaped helmet. It can attack with its retractable tail and use fire attacks. It devoured Yuichi under Kanzaki's orders when Yuichi turned down Kanzaki's offer. It was later used by Kanzaki to attack Kamen Rider Raia. Destroyed by Raia's Hide Venom. Originally designated to be the Contract Monster of Yuichi in concept arts in the event that Yuichi accepted Kanzaki's offer.
- GuldStorm (ガルドストーム Garudosutōmu?, 35, 39, 40, 43-44, 48): A red and black Fenghuang monster with a tribal personality (including the turkey-feathered headdress) that wields a tomahawk weapon. Destroyed by Tiger's Crystal Break. Another one appeared in episode 39 where it was used by Kanzaki to save Yui from Psycorogue, before used to target Kagawa's family to blackmail him into giving up the Alternate Vent. But the monster was quickly driven away by Dragredder. From the end of episode 43 to the beginning of episode 44, a GuldStorm was among the monsters that helped protect Yui. In episode 48, Kanzaki had it prevent Yui's suicide and take her where she would not interfere with the Rider Wars.
- GuldMirage (ガルドミラージュ Garudomirāju?, 35, 43-44): A green armored Fenghuang monster with Peacock feathers designed on its armor. It wields a ring-shaped weapon. This monster was destroyed by Ryuki Survive's Meteor Bullet.
- Abysshammer (アビスハンマー Abisuhanmā?, 26, 44): A Hammerhead shark monster with machine guns and missile launchers. It targeted a client of Kitaoka. Destroyed by Ryuki's Drag Claw and Zolda's Giga Launcher. Abysshammer appeared in Shinji's dream during the Hyper Video special, among those destroyed by Zolda's End of World.
- Abysslasher (アビスラッシャー Abisurasshā?, 27, 43-44): A Shark monster with two saw-like swords and water-spewing abilities. It targeted a child named Takuya. Destroyed by Knight Survive's Shippu Dan. Abysslasher appeared in Shinji's dream during the Hyper Video special, destroyed by Ouja's Veno Crash.
Buzzstingers are humanoid bee monsters that usually attack in groups. A trio of Buzzstingers first appear to target Mika, the only survivor of their attack on an ocean liner after being saved by Asakura to be used by him in order to feed his monsters. The trio used the "Trinity" attack to counter any Final Vent attack until they were all killed by Ouja's Veno Crash, Knight's Hishou Zan and Zolda's Giga Launcher from all three directions at once. Their essence was soon devoured by Venosnaker, Metalgelas, and Evildiver despite the prevention attempts from Darkwing and Dragredder. A pair of Buzzstingers appeared in episode 37 to target Eri after she came out of her coma.
- Buzzstinger Hornet (バズスティンガー・ホーネット Bazusutingā Hōnetto?, 31-32): A red-armored Hornet monster attempted to target Mika. This monster wields needle-blade daggers as weapons.
- Buzzstinger Wasp (バズスティンガー・ワスプ Bazusutingā Wasupu?, 31-32): A blue-armored Digger wasp monster that attempted to target Mika. This monster wields a sword as his weapon.
- Buzzstinger Bee (バズスティンガー・ビー Bazusutingā Bī?, 31-32): A yellow-armored Honey bee monster that attempted to target Mika. This monster wields a bow and arrow.
- Buzzstinger Broom (バズスティンガー・ブルーム Bazusutingā Burūmu?, 37, 43-44): A gold-armored Carpenter bee monster. It wields a bow and arrow in battle. This monster was destroyed by Knight Survive's Shippu Dan.
- Buzzstinger Frost (バズスティンガー・フロスト Bazusutingā Furosuto?, 37, 43-44): A silver-armored Scoliidae monster. It wields needle-blade daggers in battle. This monster was destroyed by Ryuki Survive's Dragon Firestorm.
Sheerghosts (シアゴースト Shiagōsuto?, 41, 43-47) are Dragonfly Mirror Monsters that make their presence known in the Rider War end game in the TV series (41 and after) and Episode Final, where Yui's new life is almost used up. They are actually larval Mirror Monsters, able to shoot silk from their mouths to escape or capture prey. The first two were encountered in episode 41 where one was destroyed by Kamen Rider Ryuki Survive's Meteor Bullet while the second one was destroyed by Impaler's Drive Divider. From the end of episode 43 to the beginning of episode 44, some Sheerghosts were among the Mirror Monsters that converged on Yui only to be repelled by the Mirror Monsters on Kanzaki's side while one helped to protect Yui. In episode 45, three Sheerghosts tried to attack Yui only to be destroyed by Knight's Hishou Zan and Ryuki Survive's Meteor Bullet. At the same time, a bunch of them were destroyed by Zolda's End of World. In episode 46, a bunch of them crashed the fight between Kamen Rider Knight, Kamen Rider Zolda, and Kamen Rider Ouja. The ones that engaged Kamen Riders Ouja and Zolda were destroyed by a Veno Crash / Giga Launcher combo. The ones that fought Kamen Rider Knight were destroyed by his Hishou Zan while the others were destroyed by Ryuki's Dragon Claw Fire. In episode 47, a bunch of them attacked Ryuki Survive and Knight Survive, but they managed to give the Sheerghosts the slip. Those Sheerghosts then crouched down on the ground and evolved into Raydragoons
- Raydragoons (レイドラグーン Reidoragūn?, 47-49): Blue dragonfly monsters with dragonfly tails on the back of their heads that are the evolved forms of the Sheerghosts. They have blade-like structures on their forearms and are capable of flight. They first appeared in episode 47 where they evolved from the Sheerghosts that pursued Ryuki and Knight. In episode 48, a bunch of them fought Kamen Rider Ouja and were defeated. In episode 49, the Raydragoons attack the city on the final day of the Rider Wars. One stabbed Shinji in the back when he saved a young girl. They were destroyed by Ryuki Survive's Dragon Firestorm and Knight Survive's Shippu Dan.
- Hydragoons (ハイドラグーン Haidoragūn?, Episode Final): Blue dragonfly monsters that are the alternate evolved forms of the Sheerghosts. The Hydragoons are more dragonfly-like than the other forms and are much larger. They are shown at the end being overpowered by Dragranzer and Darkraider.