I recently borrowed the collected Death of Superman from a friend. When Doomsday punches Supergirl in the face, she sort of melts (in the panel with the punch) and then takes on a wholly non-human appearance (in the next panel). I always thought she was a Kryptonian like Kal—so what's going on there?
That was the Matrix Supergirl. An artificial shape-changing life-form created by another Lex Luthor in a parallel universe. She was the only survivor after Zod and other Phantom Zone criminals killed every being on their universe's Earth. She came to Superman's universe and became its Supergirl for a time.
