Since the Season three finale "Incendium" a new love interest has entered in the picture. Her name is Flame Princess and she is the same age as Finn. But there are some major differences between her and Marceline.

  1. Marceline father is evil, while Marceline is not evil. Flame Princess' father is discribed as vert aristrocatic but cruel, and may be sadistic (for liking Jake's "death") and Flame Princess is also evil.
  2. After Finn and Marceline's battle in her first episode "Evicted!" she kissed him on the cheek after realizing how much fun she had with Finn, while Flame Princess slapped Finn after he saved her from the rain caused by PB's rockets (even though it partly was Jake's fault for the reason to Flame Princess to try and kill them).
  3. Marceline is an immortal while Flame Princess is mortal.
  4. As revealed in the two part Nightosphere episode Marceline's dad like Finn even though he isn't evil while I don't think Flame Princess' father will be pleased when he finally learns about Finn not being evil.