Adventure Time, come on and grab your friends!
This is Finn, an old friend to Marceline and Bubblegum. These three were basically the Three Musketeers. They all had different personalities but they kept each other grounded. While Marceline was mysterious and rebellious and Bubblegum was intelligent and brave, Finn was impulsive and carefree. They all grew close during their adolescence but grew apart after Marceline and Bubblegum broke up.
Because Bubblegum and Finn slept together. Yep, Bubblegum cheated on Marceline with Finn. The three of them were at a party, Bubblegum drank too much, and one thing led to another.
Finn had been in love with Bubblegum for years so he feels no guilt for what he did and neither of them told Marceline what happened. Finn and Marceline even kept their friendship going when she and Bubblegum drifted apart. Finn and Bubblegum have hit an awkward patch and limit their communication but he still has feelings for her.
Bubblegum and Marceline are eventually going to reconcile but what happened if Finn shows up and Bubblegum becomes torn between the two of them?
I'd prefer the face claim to be Luke Hemmings but any young blonde will work. Just ask if you want to use someone else. His age should stay in that range but his occupation is up to you. His member group would also be animus. And his personality is open to interpretation as well but he must be a wildcard since that's why they became friends with him in the first place.
Can't wait to see Finn around!