It is unclear as to who created Zero or when, but many believe that it may have been around the same time as X's creation.
Zero took command of the Maverick Hunters at the time of the First Maverick War, but was destroyed when saving X from Vile. X was revived during the Second Maverick War and was able to help X. Zero was part of the 17th Hunter Unit until he was given command of the #0 Hunter Unit, named after him.
At first, Zero's only weapon was a Buster Cannon, but after his revival, he has taken up a Beam Saber and is an expert in its use.

Zero has been able to help X during the later wars, and Zero was able to defeat Sigma during the Fourth Maverick War, but he still bears the deaths of Iris and Colonel. However, he is dedicated to stopping the Maverick threat.

During the Eurasia Incident, Zero was killed during the final assault on Sigma and was believed to be lost forever. However, during the Nightmare Incident, Zero reappeared when the 'Nightmare Zero' was destroyed. Zero resumed his duties as commander and as a Hunter and helped defeat the Nightmare, Gate and Sigma. After the battle however, Zero disappeared once again. Unknown to anyone, Zero sealed himself inside a capsule for about 100 years.

Zero reawakened in 22XX to find that the Earth has been restored, but the threat of the Mavericks still existed and that Signas had gone Maverick and had his army of Mavericks. Though he hasn't reassumed his position as commander of the #0 Unit, Zero rejoined the hunters.
During the Limited Maverick War, Zero had gained special armor upgrades, much like those that X had received in the past; mainly feel that it was Dr. Light who had supplied him with these upgrades. The 'Gospel' Armor increases Zero's speed and power, giving him a much stronger Buster and a special Giga Attack, the SuperNova.

Zero remains to be a part of the #0 Unit, acting more as a 'second in command' to Bolt. Zero doesn't seem interested in retaking the Unit.

X and Zero are strong friends since the beginning of the First Maverick War, and though they have been separated for over a century, the friendship has remained strong