From The Mega Man Universe
MegaMan.EXE, also known simply as MegaMan, was originally a human boy named Hub Hikari, the twin brother of Lan Hikari. However, he succumbed as an infant to a fatal heart condition known as HBD. While he lay dying, his father, Dr. Hikari, conceived of a way to combine human DNA with Navi programming to create a new type of artificial intelligence, thus MegaMan was born. MegaMan accompanied Lan on his many adventures, including the first battle against the WWW crime syndicate. Subsequently, he battled with Lan against the resurrected LifeVirus and the Zero Virus, and was also responsible for helping take down the Gospel netmafia. Later, he competed in the N1 Grand Prix as a top NetBattler and helped Lan take down the WWW's plot to frame and discredit Chaud in front of the world. He and Lan, along with all their friends, further went on to stop the WWW from attempting another plot to destroy the world by defeating the Alpha program. After their adventures, and MegaMan's near death, he competed with Lan in the Battle Chip Grand Prix and three more tournaments while fighting the powerful Neubula syndicate. MegaMan would face off against Nebula once more in a fight to take back the Net from them, but would be kidnapped and turned into the powerful DarkMega, enemy of Lan. With the help of a team of Navis, MegaMan would return to normal and defeat Nebula once and for all, before encountering the powerful Cyber Beasts and a reborn WWW. Caught up in one last bid to cause mass chaos, MegaMan would fight alongside Lan in his most dangerous battles yet, before defeating both the WWW and Cyber Beasts. We would continue to be at Lan's side at the end of the series.
MegaMan NT Warrior
As the title character, MegaMan naturally appears in nearly every episode of the anime, although his origins are different in that he is a powerful upgrade of a Normal Navi.