Alma – F.E.A.R.
Alma seems like a no-brainer, especially given how terrifying she was in F.E.A.R. She could manifest in her young form, using psychic attacks and monsters to beat down her foes. Then for the heavy-hitting attacks and Fatalities, her true self could emerge and drag enemies into another plain of horrors. Obligatory detonation of the whole arena before emerging from the ashes unscathed would also be cool.