Mega Man (known as Rockman in Japan) is a robot that was created by Dr. Light to help mankind. He is the main protagonist of the series and appears in all of the main series games. In the Mega Man X series, he is replaced by a robot known as Mega Man X. Mega Man's job is to stop Dr. Wily's evil robots from harming humans. He uses a weapon called the Mega Buster to do so, but he also uses weapons taken from defeated robots.
Mega Man was a project created by Dr. Light to, as mentioned above, help mankind. Other robots made by him were known as Robot Masters. Before Mega Man was created, Dr. Light created Proto Man, but his creation ran away resulting in Dr. Light creating Mega Man. Mega Man eventually became a fighter to stop Dr. Wily again and again.