Finella was a member of the court of King Kenneth II of Scotland in 995. Finella fell in love with Constantine, one of Kenneth's chief generals. Unfortunately, Kenneth had affection for her, and while she respected him as a friend, she did not love him back. Constantine used her to lure Kenneth away from the safety of his guards, and assassinated him. Finella was grieved by this, and once she realized she had merely been used- as Constantine quickly abandoned any pretense of loving her to have Princess Katharine- she grew to hate Constantine.
Eventually, she was angry enough to betray him. Siding with Katharine, the Magus, Tom, and Mary, she met him at the place where he had killed Kenneth, ostensibly to beg for his favor. The Magus gave her a flask of drugged wine, and a small protective stone to hold in her mouth when she drank it; Constantine slumped into unconsciousness upon drinking it, but Finella did not. Finella accompanied her four allies to Avalon, but when the Magus could not take the Grimorum Arcanorum to the isle, she offered to take it back to the outside world and protect it. She was accompanied by Mary.
Greg Weisman's Master Plan
Finella and Mary ran from Constantine for two years, but were saved by a time-travelling Brooklyn in 997 on his first adventure. Finella and Mary were transferred along with Brooklyn to America in the 1970s. Presumably, Finella and Mary are still alive in the present day.