Olivia (Izabella Miko) is the hidden main villainess from Supernatural episode 10.06, "Ask Jeeves" (airdate November 18, 2014).
Olivia is a shape-shifter and the daughter of the immensely wealthy Bunny LaCroix and an unnamed male shape-shifter. Olivia's father was killed by Bobby Singer while he came after her, but Olivia herself was spared by Bobby, on the condition that she remains locked in the attic of her house for safety reasons.
Despite being spared by Bobby, Olivia sought revenge on Bobby, and after Bunny passed away, her butler, Phillip (the only one who knew the truth about Olivia) released Olivia and gave her a job as a maid. Olivia, however, took the opportunity to go on a killing spree, beginning with taking the form of her deceased mother and killing fellow maid Colette. She continued taking the form of deceased family members and killed Stanton and later Phillip, who's form she assumed. Olivia's evil secret was found out by Sam and Dean, leading to a confrontation between the villainess and Sam. Olivia stated that she planned to kill Bobby, but after being told by Sam that Bobby died three years prior, Olivia decided to do the next best thing and kill Sam. She is later shot (repeatedly) and killed by Dean.