Marcus Keller

Marcus Keller was a boy of Germanic origin and one of the antagonists during the conclusion of the My New Life as a Devil of Leviathan and the beginning of the Second Arc of the story. A stray devil who became a grim, he was once a servant to the Orias Clan until his defect and he joined the Stein Nest until he betrayed them as well and joined the Raynare Group, with the goal to aid in stealing Asia's sacred gear, get revenge on Ichiro, and to destroy Kuoh Town, the town belonging to Rias Gremory.

Appearance
During his first appearance Marcus takes the appearance of a monster, he appeared to be 16 or 17 years of age with cold blue eyes and elf-like ears, with long wild and messy black hair and with what appeared to be a helmet that resembled a dragon's skull with four horns on his head. Marcus also had a mouth opened on his chest with sharp rows of teeth and black scales on his legs and webbed feet. Wearing no clothes, Marcus had four black spiked collars on his arm.

After Raynare forced Marcus to undergo a purification ritual his appearance changed to the point where he was able to sneak attack Ichiro during their second encounter, him going back to his original form, his black hair reverting back to blonde and looking more clean and going to his shoulder. He also loses a few inches and while his eyes stay the same, his teeth lose most of their sharpness however his canine still retain their sharp edges.

Personality
Marcus takes the appearance of a very intelligent, kind, and caring individual. He possesses enough charisma that he was able to manipulate numerous individuals into allowing him into their groups (although his power as a mutation rook is also known to be a good motive). He is a individual that can be described as a narcissistic, taking advantage of others and even abusing others if he doesn't get his way, shown by how he murdered his former master and the woman he was in loved with. He is also shown to believe him to be superior to others, even if evidence is shown to the contrary.

He is shown to be a individual with no sense of loyalty or even love, as he murdered his former master, despite how kind he was to him, and murdering the woman he states to love because she didn't share these feelings back. Ichiro later jokingly calls Marcus a hypocrite, while he "rattles on" about how Ichiro is a slave and not free, he's a slave to his own anger and hunger.

When he is angered he is shown to pursue the source of that anger relentlessly, shown by him pursing Ichiro due to the embarrassment and humiliation he felt during his first fight. This anger and humiliation was also the reason he murdered his former teammates out of blind outrage. Ichiro later describes him as his dark aspects minus Dark Ichiro's intelligence.

History
Marcus was a boy born between a fallen angel mother and a human father with devil ancestry. Due to his mixed bloodline he faced hatred and discrimination from other fallen angels. He later ran away from his parents where he met Eli Orias, a young devil from the Orias Clan, at the Underworld capital. Marcus became his first servant, taking his mutation rook piece. The two then spent a few years together finding more allies for Eli's peerage and training together.

Marcus then fell in love with Eli's bride, a noble woman from the Sallos Clan, who didn't share his feelings, when Eli confronted him about his actions towards her Marcus flew into a blind outrage and killed the both of them eating pieces of flesh from the two devils. He then escaped from both the Orias and Sallos clan's wrath and went to the Underworld, landing in Germany where he consumed dozens of humans becoming a creature known as a Grim. He later met and joined the Stein Nest, becoming the second strongest individual behind Amera.

Powers and Abilities
Hell Light: A special ability Marcus created by tapping into his mother's abilities and his father's ancestry, by infusing his demonic energy with light he can create a weak form of the Satan's clan original ability, dark light. While not as strong it is shown to be capable of burning both angels and devils if touched.

Demonic Hands: A ability he gained by eating the flesh of the Sallos girl, he gained her ability to summon black shadowy demonic hands to aid him in battle.

Minor Power over Astrology: A ability he gained by eating the flesh of his former master, he gained his ability to manipulate gravity to a minor degree, capable of summoning small compressed balls.

Immense Strength: Due to being a mutation rook, Marcus is shown to possess immense strength.

Immense Durability: Being a mutation rook, Marcus is very durable, able to endure being stabbed by Ichiro and getting up and murdering the rest of his peerage. Over the course of a battle Marcus states that his durability only gets stronger, a special trait he possesses as a mutation rook.

Flight: Marcus is capable of flight possessing one devil wing and one fallen angel wing.

Trivia

 * While Marcus isn't as big of a glutton as Amera they are both to be kindred spirits in a way.