Bubbline

"The fanfiction actually began to seep into the show. The 'Bubbline' relationship between Marceline and Princess Bubblegum was never intended, but because it was a fan favourite, they're focusing on it a little bit. […]

I don't think there was a singular emotion to describe the moment Bubbline became canon.

Still, intrigued by Bubbline, I'd occasionally check in on the fandom to see what was up with their relationship.