Cover of Soul Cartel volume 1
Soul Cartel follows Si-hun Cha, a young Korean boy who is able to see the spirits of the dead. He is taken care of by his older sister, Su-In Cha, and comes from a family of powerful mediums and exorcists. Si-hun Cha himself is lacking in any spiritual abilities whatsoever aside from his ability to see spirits. Despite this, he tries to help his sister catch weaker wayward ghosts. When chasing after one of these ghosts, he runs into a very powerful demon. The demon claims to be the Mephistopheles, or shortly, Mephisto, who wants Si-hun Cha to teach him of the modern human world and bring him books about his exploits. He states that if Si-hun Cha tries to run away, he's done for. Si-hun Cha does have the common sense to consult his sister (without revealing that he's actually caught a powerful demon's attention), but he quickly learns that even his sister would be no match for Mephisto. After Mephisto finishes a copy of Faust he reveals that he came to earth because Faust has been reincarnated. Faust was a human who'd claimed that he would not be corrupted by Mephisto's temptations, so they'd entered a contractual wager, with God acting as the judge. However, according to Mephisto, God had unfairly intervened at the last moment and cost Mephisto his victory.  
Si-hun Cha (차시훈)
Si-hun Cha is the main protagonist of the series. Although he is the main hero, his role in the series so far has been severely limited. This is because of his lack of strength and skills. But rest assured, he is still young and in training. He has shown exceptional advancement by gaining a special move known as the 3rd hand, a super punch which is strong enough to take on an A-class demon.
Su-in Cha (차수인)
Su-in Cha is one of the main characters of the series and is the elder sister of the protagonist Shi-hun Cha. She took up being a spirit medium after her parents vanished during a mysterious fight with a powerful being from the underworld. After a failed attempt to rescue them from the Underworld a few years ago, she was quietly doing her job guiding spirits to the next world when she came to know that her brother had gotten into a contract with the Archdevil Mephisto and would have to go the underworld. Upon hearing this, she decided to come along to protect her brother.
Faust is a Human who due to being reborn has decided to re-engage in his battle with the Archdevil Mephisto as a means of ensuring humans are safe. Due to 'said interference from god, Mephisto has claimed he never really lost. In this battle Faust will utilizing God's Papyrus a tool with divine powers from god that can do anything but be used for evil unpure reasons. Faust has used on numerous occasions to subdue Mephisto to a degree.
Mephistopheles (메피스토 펠레스)
Mephistopheles (more commonly known as Mephisto) is one of the main characters of the manhwa and is the first Archdevil shown. He is also known as the Archdevil of combat.
Beelzebub is the second Arch-devil to appear in Soul Cartel who consider Mephisto as a rival.He aims to overturn the Arch-devil hierarchy. He is also known as the Arch-devil of slaughter.