Finn/Fionna Parallels

Finn's physical degradation is being paralleled by the degradation of Fionna's moral backbone.

She's losing herself. She agreed to work with her friend's abusive, power-hungry mother (thus pushing him away), and is obsessively chasing this fantasy with Phelix despite him having a girlfriend. She doesn't recognize the effort her friends put in to the event. She is single-mindedly chasing this fantasy of being a hero, while, in reality, she is hurting those she is trying to "save".

The threat looms — both of a physical death (Finn), and a symbolic death of the self (the "Fifi" future).