When Marshall Lee first met Fionna (and Cake) she was a naïve little 13 year old with a stretching cat. She ran around in a bunny hat and skort. He enjoyed pranking her and having her help with his songs. She has a surprisingly good singing voice, like honey.
The next year Fionna was 14 and beginning to hit puberty. Poor Cake having to deal with that 24/7. He couldn't see a difference though, he still sees the adolescent preteen child with the knack for killing things. Well, she still is a monster slaughterer but the preteen thing has changed. To him anyways.
15. Marshall Lee realizes on her birthday that she is actually ageing. Getting older. He realizes that she will die eventually. Just like the others that he has met in his undead life. He decides to spend as much time as he can with her, she's different than the others and he can feel it, he just doesn't know how yet.
Okay so this was just the prologue and the next 2 years will be about one chapter each. Probably. After that it will be a much slower time change. Can't wait to write the rest!