She doesn't love him, as she loves Princess Bubblegum, but different, as they still have their 'little adventures'.
This was made more explicit in Rebecca Sugar's original draft of the song, widely available on the Internet.
These details have yet to be explained.
This page describes Marceline 's of vampires, Marceline often teases Jake and intentionally horrifies him. The two are shown to be able to clash easily, as seen in "What Was Missing," when Marceline sings "I'm.