In Adventure Time Stuff, Marceline Is Front And Center As Indisputably The Single Most Popular Character,

in adventure time stuff, Marceline is front and center as indisputably the single most popular character, widely loved within and out of the fanbase; she's kind of a distillation of the series' wacky tone because she is unironically a vampirized half-demon child of Satan Stand-In, a heavy metal rock star who's dated a mad scientist princess, her adoptive dad is arguably one of the most powerful entities in the setting and a terrifying ice wizard who survived the apocalypse through cunning BEFORE he was consumed by his curse, and the greatest hero in the land who is otherwise a mundane human won her friendship by punching her so hard she went 'FUCKING SWEET, WE ARE NOW BROS FOR LIFE'

but the thing about Marceline that is arguably the single most essential aspect of her character, and not something that is apparent from the joke or memes around her

is how nice and genuinely sweet she is.

Yes, she's a badass. Yes, she is probably the single most powerful character in a conventional sense among Finn's friend group, to the point that she is less present in serious situations because her picking a side would trivialize just about anything short of the Lich or the apocalyptic arrival of Golb.

But as we see more of her character, we see that she goes out of her way to actually avoid seriously upsetting people. The most recurring motif of her songs is that while they sound threatening or spooky, she says them in a very sweet and loving way that makes them too silly and harmless to take seriously. At her core, that's kind of who she is; she's honestly a genuinely nice person who deliberately distances herself from people because she outlives everyone she cares about, one way or another, and she pushes people away so she doesn't have to see it happen, but she can't help but get close to someone.

In a lot of ways, Finn helps her grow out of this mindset, and the relationships she establishes through him, or rekindles because of him, help her grow out of this. But even then, she has no real mean-ness in her

ironic, given the "MARCELINE WHY ARE YOU SO MEAN" song, but she gives her own answer right there:

"I'm not mean/I'm just a thousand years old and I lost track of my moral code"

Tellingly, when we see her as a young girl (and before meeting Simon, likely almost right before she met him), she has internalized the idea that she's a horrible, awful monster who scares people away. That she loses everyone around her, because SHE'S a scary monster. We see the source of her tendency to push people away, because that way she has some kind of control over it, and otherwise she thinks its inevitable.

But she can't help but get close to people anyway, and in many ways, her relationship with Ash is a very clear example that she is NOWHERE near as violent or ferocious as she looks. Their relationship openly has her abused, meek and quiet, and she is a nigh-unstoppable soul-eating monster that NO ONE could reasonably fight. Even so the closest she gets to actually lashing out against a guy who sold her most cherished possession (and the last remaining reminder of a father figure who can't ever come back) is to just end their relationship instead of lashing out against him.

Much like the reveal of her relationship to Ice King (which interestingly also maps to his slow recovery from being a villain at all, to the point that around the time this becomes a big aspect of his character he is functionally not really a villain, though still a danger to himself and others), its a slow burn. Showing that she's by far one of the least malicious beings in the entire setting, and she could be a hero.

And the answer to that one is: because she used to be. And that part of herself isn't as long-gone as she would like to think.