Ahaha, the next girl will blow your suspicions out of the water making them both prime canidates. Trust me. I will say this though, the girl is a very important character.
Well, there is this main plot point where Serena gets out of her insane state, disorientated but entirely lucid for the first time in who knows how long... before getting killed by Vioretta so she can summon the purple butterfly back into the human world.