Mags is an assassin working for The Lethe Cult. She is highly skilled in her work, previously working for several criminal organisations in Baldur's Gate before she was tasked with assassinating Paladin Primrose. They spoke a long time before Mags decided to throw her lot in with the Lethe Cult.
She is unsure if this 'Lethe' thing is fully legitimate but Primrose could order her to invade the Nine Hells and Mags would do it.
She was killed by Kuno but luckily, got better.