N'awww... Baby Marceline...
Ok I imagine that Marceline and Simon travelled around for a while, but after a while Simon realised that he was going to end up hurting Marceline and so he had to leave.
But Marceline waits for him for a long while before coming to the realisation that he won't be returning and she has to continue surviving by herself.
AHHH ADVENTURE TIME WHY MUST YOU MAKE ME FEEL THESE THINGS :c
As always, comments and critique are much appreciated!