Kamek
Kamek was King Bowser's guardian (and possibly adopted father) and the ultimate enemy and the main antagonist in the game Super Mario World 2: Yoshi's Island. He kidnapped Baby Luigi and tried to kidnap Baby Mario, but Yoshi and his variously colored friends protected Mario and saved Luigi after defeating Baby Bowser. He may be Yoshi's biggest foe and enemy.
Kamek is of the Magikoopas, an offshoot of the Koopa Troopa species that can perform advanced magic. Like most members of his class, he wears big spectacles and rides a broomstick. During the events of Yoshi's Island, Kamek attempted to block the Yoshis' progress by swelling enemies to dramatic size using magic. Unfortunately for him, the Yoshis beat all of these bosses. Kamek's minions, called Toadies (not to be confused with the friendly Toads) carry Baby Mario off to their master if Yoshi lets him float free for too long.
Kamek has made a few cameo appearances in other games, such as Tetris Attack and Mario & Luigi: Superstar Saga. In the latter, he was referred to as "Psycho Kamek," and actually helped out by hypnotizing Luigi into thinking he was Mario. This Kamek was originally suspected to share the name, but in Mario Party Advance it is established that Kamek is not quite as evil as his master and is even at peace with Yoshi now. But in the end credits to Mario Party Advance the whereabouts of Kamek show that he "Forgave Bowser and joined his side".
He also appears in Mario Kart Super Circuit on the GBA at the end of Bowser Castle 3. It is popular fanon to apply his name to other prominent Magikoopa characters in Mario games, such as the Magikoopa bosses in Yoshi's Safari and Super Mario RPG, or the Magikoopa who was originally intended to be a playable character in Mario Kart 64 (he ended up being replaced by Donkey Kong).
Interestingly, Kamek's character may have been conceived earlier. The Super Mario World cartoon episode "Ghosts R Us" featured a Magikoopa named Wizardheimer (or Wisenheimer, as Mario and company keep calling him by mistake), who claimed to be "the most dangerous Koopawizard of them all". Then the Super Mario Adventures comic series that ran in Nintendo Power during 1992 had Bowser confiding in a Magikoopa who looks, acts, and could quite possibly be Kamek.
Kamek more recently appears as the antagonist in Yoshi Touch & Go. The game essentially follows the same storyline as Yoshi's Island, and therefore Kamek is again trying to kidnap Baby Mario and Luigi. Yoshi again blocks his progress by dodging or defeating his minions. Among those returning from Yoshi's Island are the Toadies, who now are given the job of retrieving Baby Luigi, and must be stopped by Yoshi.
Kamek also makes a cameo in "Mario Kart: Double Dash!!". In the Baby Park course, his image can be seen alongside the words "Kamek's Magic Show".
Kamek's roles in Bowser's plans have dwindled, while Kammy Koopa seems to have replaced him as Bowser's personal Magikoopa. Judging from his most recent appearances, Kamek is apparently retired (though he seems to help Bowser every now and then). He made an appearance in the game Mario & Luigi: Partners in Time supporting Baby Bowser like in his first appearance; and also being the boss of the Yoshi's Island area in the past. (One popular fanon bit of info is that Kamek and Kammy Koopa are siblings.)
After Mario & Luigi: Partners in Time, Kamek was featured as a boss character in the Nintendo DS platformer Super Princess Peach, known in the Glossary as "Giant Kamek." He is the boss of Giddy Sky, the seventh world in the game. Upon defeating him you rescue Luigi, rather than Toad like with every other boss in the game (Excluding Bowser). (The only difference in rescuing Luigi is that is says "You rescued Green man?!") This goes to show that while he may not work with Bowser regularly anymore, he still does his dirty work from time to time.
In the Mario Party series, characters can obtain and make use of a Kamek Orb item. When summoned, Kamek will cause up to three of an opponent's Orb spaces come into possession of the player whom the Kamek Orb originilly belonged to.
Although the Yoshi's Island character is referred to as "Kamek" in the U.S. release, in Japan, "Kamek" (カメック; kamekku), a pun "kame", the Japanese word for turtle, is the name used for "Magikoopa". That is to say, the character known as "Magikoopa" in the U.S. has always been called "Kamek" in Japan.