Meggan was abandoned by their parents being a baby, when, unwillingly, her mutant power -morphing- appeared, covering her in fur as reaction to the bitter cold.
She was adopted by a group of gypsies, but Meggan's shape change according to how she feels or how the people around think she should be, so she took a mostruous appearance and used to spend the time hiding from humans.
When she grew up, she lived in an abandoned building, where she met two youngsters who were not ashamed of her and became their friend, but Captain Britain met them all, and thinking she was a menace, overreacted, the building was destroyed and a boy died because of this. Maybe to deal with his guilt, Brian took Meggan to Braddock Manor, were she stayed for a while, but soon, when a friend of Betsy (Psylocke), the twin sister of Brian, met her, they all discovered her true form, a extremely beautiful young woman. In time, Brian fell in love with her and even proposed her marriage, but they haven't married yet.
Due to her isolation, Meggan is extremely naive and innocent. She even believes everything that they say on TV Now, she's able to control her power when she wants to (although sometimes she still changes shape unwillingly when she's distracted or worried)
During a trip with Rachel, Meggan discovered her true self : she's a fairy, not a mutant (Tuatha de Danann, according to Celtic folk). This has a secondary effect : when her surroundings are extremely corrupted, she gets ill -as we saw during the Cross Time Caper Saga.