These are all of the main/semi-main characters in the first three Megaman series.
Megaman X was Dr. Light's final creation before he died. He was able to independantly think and make his own decisions, making him more human than any other machines built before. When Sigma went berserk and took Mavericks with him, X joined the Maverick Hunters despite having a grudge against violence. Each time he had defeated Sigma he became more unsure of his place and if fighting was causing more problems than it was helping.
Zero was once the leader of the Maverick Hunters (taking over after Sigma went Maverick) and X's best friend. Zero's past remains a mystery, although it has been hinted that he was Dr Wily's creation to annual Dr Light's creation of X (Sigma hints this a few times). At first Zero was a Maverick but he ended up transferring the Wily Virus to Sigma which had caused him to go Maverick.
Sigma was Dr Cain's most advanced prototype reploid (and leader of the Maverick Hunters). After his narrow victory against Zero Sigma had begun to change, he started to become obsessed with superiority and power. Sigma soon went Maverick convincing most of his Maverick Hunters to join in his rebellion. Although X had defeated him and his Mavericks numerous times he would always reappear in more and more powerful bodies (he stayed alive each time as he was a virus that thrived through computer systems).
Dr. Light was the created of the Original Megaman and rival of Dr Wily. Before he died he created Megaman X and sealed him away, hoping one day he would awaken and there would be peace between humans and robots. To assist X in his fight against the Mavericks, he had hidden upgrade capsules all over the world.
Dr. Cain was a great scientist who studied plant life. He had discovered Dr Light's Lab and found the capsule containing X during one of his digs. Amazed by this Dr Cain used designs from Dr Light to create Reploids. When his best Reploid, Sigma, went Maverick Dr Cain had felt responsibility for the ensuing wars between the Mavericks/Maverick Hunters and regretted continuing where Dr Light had left off.
Dr Doppler was a reploid scientist who was made to support humans in the creation of more reploids. He had created the anti-virus to stop the spread of the Virus and Mavericks. He later created a city where humans and reploids would peacefully live together but ended up being infected with the virus himself that he had reated a cure for. His goal as a Maverick was to create a newer, more powerful body for Sigma to control.
Vile was one of Sigma's top henchmen. When he first encountered X he had easily defeated him, but on his second encounter he was destroyed by X thanks to the help from Zero. Later on Vile was rebuilt by Dr. Doppler but he he was different as he had gone crazy, only bent on getting revenge against X. He was once again destroyed by X but he had vowed to haunt X forever.
Six months after X had first defeated Sigma, Serges, Agile, and Violen had rebuilt and taken control of the Maverick force. They would watch X take down Mavericks and wondered how Sigma could be defeated by someone so weak. They each tested out X's strength encountering him gambling parts of Zero's body. After losing against X and losing Zero's parts they became furious and bent on destroying X. The next time they fought X they had bigger, stronger bodies but were still defeated anyway, and revealed that Sigma was still alive.
Dr Doppler created Bit and Byte to constantly harrass X to slow/stop him from putting down the Maverick outbreak. After losing to X the first time they used technology to combine themselves into one in attempts to take X out (which failed).
