I've got an idea about the Lost Soul. What if it's no one in special, but it was originally a demon or something that is needing a body. Perhaps Trevor tries to fight the demon whatever was inside The Lost Soul and as he lost he entered that body, while what was into the Lost Soul entered Trevor's body and became Alucard. That's why the Lost Soul talks to Trevor BUT no Simon, 'cause that would be telling "Hey I have the voice of Trevor, yep your father is now a boddo doll! Hi son!"
Thoughts?