Doppelganger

The doppleganger boss is fought in two areas in the game. The first fight is in the Underground Waterway, where his weapon is a mockery of the Vampire Killer (the copied whip glows red) and has a blue body. The second fight is before the second floor of The Pagoda of the Misty Moon. This time it glows red, has more HP and attacks with it's fists.