Marsha is Marceline's younger sister. The two have been best friends-rivals ever since The Secret Sister. In Secret Sister, the two fight, both ending in a tie.
In the universe, Simon stops him by using his powers to stop his bomb from hitting Earth, but is killed in the process when he was crushed by the nose of the bomb. Distraught by this act of bravery, Marceline protects his dead body for the next thousand years, even avoiding becoming a vampire just to protect him and, more importantly, the .
It seems that Marceline and have a fairly friendly relationship, she is seen playing it in "Evicted!" and comments on how she thinks it's cute. Also in the episode "What Was Missing" Marceline assists BMO and proceeds to call it "baby."
It was later revealed in the episode "" that Princess Bubblegum and Marceline do have feelings for each other. Bonnibel came to watch Marceline's concert because she loves her music, Marceline invited her backstage. When Marceline sung an extended version of I'm Just Your Problem and Oh, Bubblegum, Bonnibel realised that Marceline also had feelings for her and they started a secret relationship. In "" they finally revealed their relationship to the others, in "" Marceline took the step to introduce Bonnibel to her father. Hunson eventually approved and they could be open about their relation.