Roll is a kind-hearted humanoid "Robot Master", what could be summed up as a type of android, created by Dr. Light to be a house keeper.
Roll is a kind-hearted robot, who would be described as trying to see the best in everyone. It is heavily implied in some games that she took care of the Megaman 1 Light Bots to a degree. Perhaps like an older sister? However, despite a rather passive and forgiving nature, Roll can be provoked to action under the right circumstances and can experience a rather terrifyingly contrast level of aggressiveness to her normal attitude when pushed into a situation that calls for it. She also sometimes displays an uncanny level of intelligence and maturity for someone who looks so young. Mostly when something calls for such intelligence or maturity. What can I say. She's a robot. Roll has a deep seated dislike of "filthy" things, and an almost compulsive desire to clean things (or people <___>) that aren't clean enough for her liking.
Roll Light was created to help Doctor Light around the lab. It's probably accurate to say she was around during every classic Megaman game ever made. Due to an inherit cowardice and the fact that, well-... someone had to stay and help Doctor Light, right?! ... Moving on, due to that, she rarely if ever got involved in actually saving the world with Mega Man beyond supporting him from the sidelines. That isn't to say it never happened. (She was originally intended to be a playable character in the games, sort of like Luigi to Mario in Super Mario Bros.) She managed a shop in the later games that unfortunately required "various parts" (See: Bolts, Screws, whatever) to finish parts for Mega to use. It might be assumed that around the future time Wily traveled to steal Mega and turn him into Quint that Roll was separated from Mega permanently. Also, from Roll's creation until Megaman 9, Roll was the /only/ female Robot Master for like ten or eleven years