#we've all known that obviously but simon and marcy always hesitate to call each other father and daughter

[petr1kov]

i think that raising marceline is not something simon gives himself much credit for due to the terrible circumstances surrounding all of it. he feels guilty for using the crown and for hurting marceline's feelings with his transformation in the long run, so he looks back on his past with her always with the feeling of wishing he could've handled it differently, better (although how, exactly, he's not able to say, as we see when he's left without an answer when fionna questions him about the need of using the crown to keep marcy alive).

this episode with evil marcy, however, made it clear to him that he was way more important to her than he gives himself credit for. he found her when she was a vulnerable child and tried to protect he not only physically, but mentally as well; he made sure to sing and joke and play with her, to keep her as shielded as possible from the horrors of the wasteland surrounding them. he tried his best to give her a real childhood, in short. but left to the indoctrination of a terrible person like the vampire king, that same little girl wasn't allowed this, and was instead molded into someone completely different and much more miserable, still in the shadow of her terrible 'father'