Roll

Roll is a kind-hearted humanoid Robot Master created by Dr. Light to be a house keeper, helping Dr. Light around the lab. She is the sister of Mega Man and Proto Man. Roll is one of the few female characters in the Mega Man classic series, and the only female Robot Master until Mega Man 9, where Splash Woman was introduced into the series. In spite of her minor roles in the games, Roll is perhaps one of the most special and well-known characters in the entire Mega Man series, and remains a fan-favorite to many. She is especially well-known as the first female character ever in the Mega Man universe. She is also special in a sense that she, with her counterparts Roll.EXE and Roll Caskett and appearances in other media, combined, has made the most appearances of any supporting character in the Mega Man series as a whole.

(Source: The Mega Man Knowledge Base Wikia)