Bubbline

Bubbline is the femslash ship between Marceline Abadeer and Princess Bubblegum from the Adventure Time fandom.

Canon
Their first interaction came in the episode "Go With Me." They seemed hostile towards each other and mocked each other. The shipping began in the episode "What Was Missing", in which Marceline sang the song "I'm Just Your Problem", which hinted at a past romantic relationship between the two characters. Furthermore, it was revealed that Bubblegum's most treasured item was a t-shirt that Marceline gave her, which she wears as Pajamas.

In the episode "Sky Witch", Bubblegum is shown in this T-Shirt and smells it deeply before going into her closet, which has a picture of her and Marceline in it.

They were also recently seen holding hands and wearing each other's clothes in the episodes "Seventeen" and "Marcy and Hunson"

Fanon
It's very much possible that PB and Marceline could've been past lovers. It's also been mentioned by the cast in interviews about whether or not Bubbline was actually cannon.

As mentioned before, it's revealed that PB's most meaningful item was the shirt that Marceline gave her before. This is widely used in all fanfictions, as it shows that PB actually does care about Marceline and even wears her shirt to sleep and sniffs it when she wakes up.

One of the other things is when Marceline has a dream where her and PB are living together as an old couple. It's widely interpreted as Marceline dreaming about what future they could/will have.

Fandom
AO3

Trivia

 * The reason Marceline and Princess Bubblegum were hostile towards each other during the beginning of the series is because Bubblegum distanced herself by dedicating herself to her work over everything else. This was revealed in episode "Varmints".
 * Princess Bubblegum wears Marceline's old shirt to bed.