Here is my Marceline theory:

Ok, I think that Maceline's Mom was human, and had Marceline before the GMW. I think that Marceline's Mom was a human because in the episode Memory of a Memory, when marcy was a child, she was not seen flying and she didn't have bite marks. During the time when she was a kid, she resembled a demon (Pointy ears, pale skin)- Human (Huminoid Features, such as light skin.) hybrid. I think that marceline's mother died from the radiation of the mushroom war, and Marcy could survive because she was part demon. I think that when Marceline was around the age where she looked 13, she got bitten by a vampire, allowing here to fly. She still kept her featues and her demon powers (Which would explain why she can shape shift into things other then a bat.) The person that bit her was the Vampire King, and he made her is queen. She then killed him. And that is my theory.