Marilla was a member of the Inhumans and had lived there for many years. She soon became the nanny of Crystal and Quicksilver's child Luna.
Marilla was created by Bob Harris and Steve Epting and first appeared in The Avengers Vol.1 issue 343 (1992).
Major Story Arcs
Living with the Avengers
When Crystal became a part of the new Avengers team, Marilla and Luna traveled with her and made Avengers Mansion their home. She had a lot of trouble with the mansion's buttler, Jarvis. Even the almost always polite Jarvies cleary made his dislike of her noticible.
Eventually Marilla was killed by a mind-controlered Tony Stark when she discovered his plans to take out the Avengers. Not much was heard about mourning her death, but that was possibly because of the events that followed, with Iron Man turning against the Avengers. Years later, Marilla was honored as one of the dead member of the Avengers.