Well first Comparison is...........

Fionna-Susan Strong = well Fionna might have amnesia and to explain the skull with Fionna's hat on it Fionna was fighting the lub glubs and to put them off she put her hat on it because her hat smells like her (<--duh)

Marshall Lee-Vampire King (that Marcy killed) = well Marshall wanted a bride and because he was also from the Nightosphere he found a perfect one, Marceline when she visited her father.. then Marcy killed him and she became the Vamp Queen

Flame Prince-Flame King's brother (that he killed) - well when Flame Prince was the heir he had a younger brother the future Flame King but since he was the younger brother he wasn't the heir so he killed Flame Prince before he was crowned.

Prince Gumball-Uncle Gumbald = the dissapearance of Fionna and the death of his friends greatly shocked him, so in their name he began to train and even took down an entire forest (that Bubblegum talked about before Finn and Jake found Susan).

Cake and Monochromicorn = they had kids and because of the dissapearance of Fionna she couldn't bear it so she just left the Grasslands and Monochromicorn took her to his dimension (right next to Lady's Dimension).