Zero's mother died when he was born and his father was known throughout the world as a fearsome demon hunter and assassin, raising his son to someday follow in his footsteps. 

At the age of 7, Zero was attacked and possessed by a demon he and his father had been hunting known as Abaddon.
His father, managed to seal the demon into the point of entry; Zero's left eye. As complete removal was impossible, an eye-patch was created with powerful seals embedded in it which would lock Abaddon's presence away entirely. 
As his body was still running rampart from the demonic powers flowing through his body, Abaddon forced Zero to kill his father, piercing his body with his hand while placing the eye-patch on his sons face.

Zero wandered Ryner for 3 years, living off the land as something of a feral child when Hunter found him and brought him back to Pulse.
Although still quite young, Zero's abilities both as a projectile meister and an assassin made him a valuable asset to the team and he was raised as Hunter's son within GRAVE.

Zero is somewhat serious in nature, with his mind always focused on the next objective. He is the team leader for the Reapers and is responsible for all tactical decisions when in the field.

Powers & Abilities:
Zero is the Lord of the Projectile class, using his twin pistols "Lux" & "Ater" to shoot concentrated spiritual energy in order to do battle.
Zero's Overdrive "Ragnarok" disintegrates Lux and Ater into finely compressed spiritual energies, taking on the form of thin red wires attached to his finger-tips. These wires are able to either restrain an enemy or slice them clean through, depending on the Zero's will.
Throughout the story, Zero also attains an extremely rare and superior ability known as an "Overdrive Plus"; in which Lux and Ater transform into a massive scythe with a highly destructive blade made from pure energy; multiplying Zero's strength ten fold.