Another friend of her is Princess Bubblegum, they call their friendship 'friendly rivaly'.

In 'What was missing', it is revealed that Princess Bubblegum's stolen item is a t-shirt that Marceline gave to her some time ago. Bubblegum says that the shirt means a lot to her and admits that she sleeps in it, causing Marceline to blush (she only does that when she's feeling emotional). In "Sky Witch" Princess Bubblegum is shown sleeping in the shirt from Marceline, before waking and smelling it deeply. It is also shown that she has a picture of them in her closet, further indicating that she harbors sentimental feelings of some sort for Marceline.

Though they were initially enemies, Finn and Marceline soon befriend one another. Even when Marceline acts as a bully in her debut episode "Evicted!", she is impressed by Finn's brave aggression and defiance and chooses to kiss him on the cheek after they fight. Both Marceline and Finn share the same spark for adventure and love of danger and excitement. Marceline is very good friends with Finn and seems to trust him and be very comfortable with him. Jake is most of the time scared of her.