Zero

Zero was originally a Pokemon known as Zekrom, though he became human when he ingested the Blood Pearl. The only remaining facet that points to his non-human past are his eyes, which kept the red sclerae and white irids that he had in his Pokemon form, so he usually covers them with sunglasses. Losing the powers he had as a Pokemon, he had to learn human magic and he has become quite adept at it. He is Nero's most powerful familiar in terms of sheer power output, and is also one of his most loyal.

Appearance
As a human, Zero appears as a very tall man in his mid twenties with short black hair that has a single blue lock near the back of his head. Due to his odd eyes he often wears dark sunglasses, and he has a penchant for wearing black coats and black clothing in general.

Personality
Zero is a very serious man, and doesn't treat things like trust lightly. Often he will summon himself to stop Nero from showing his true power to one he deems unworthy of facing it, and takes them on himself. In actuality, he is quite similar to Jackie and acts like Nero's bodyguard.

Personal Traits
Zero has two traits which point to his non-human origins - his eyes and the blue spot on the back of his hair. Though the spot could be excused as dye, there is no excuse for his red and white eyes given that the red is where the white should be and irids should never be white in the first place.

Physical Abilities
Very Strong: Zero has a lot of natural strength, though one would expect him to be strong given his large size.

Magical Abilities
Incredibly Strong Magical Power: Zero's magical powers has been noted to be very strong and very potent, being able to charge the air around him with electricity when he unleashes it, in turn being able to shock people in a manner similar to static electricity if they come into contact with a conductor.

Magic Color "White": Notably even though Zero has a very "black" theme going on, his magic color is actually white.

Bolt Strike Magic
Zero's personal magic style is named after his strongest attack from when he was a Pokemon. He can generate very powerful white electricity around him which he uses in combination with his natural physical strength to devastating effect.

Trivia

 * Zero's name parallels Rei's - Rei means "Zero" in Japanese.