Cestis is first seen attempting to break into The Lux in an attempt to gain control over the portal that Lucifer has opened. She is initially confronted by Mazikeen and eventually killed by Jill Presto.
After Cestis’ original death, she is resurrected by The Basanosand sent to assassinate Elaine Belloc. To sneak up on Elain, Cestis kills and eats Elaine’s father in order to take on his form. However after the completion of her mission, Cestis remains trapped in Mr. Belloc’s form and finds herself overcome by his feelings and memories, slowly actually becoming the man she killed.
All appearances of Mr. Belloc after Lucifer #21 are actually Cestis in disguise.