The Lich is very much the exact opposite of Finn. The Lich was born from the Green Catalyst Comet that struck Earth and killed off the dinosaurs and seems to represent Mass Extinction while Finn is the Reincarnation of the Blue Catalyst Comet, which incarnated as a butterfly (a symbol of rebirth). While the Lich is pure evil, relishes in his cruelty and wants to exterminate life, Finn has a pure heart and is more than willing to fight against the bad guys.
Scorcher Hyper-competent fire power user to counter Ice King's inept ice magic.
Ice King to Magic Man. Both are antagonistic magic-users with insanity rooted in the loss of a loved one, but while Ice King tries to make people love him, Magic Man's goal is to be the most hated man in existence.
Ice King to Lemongrab 1. Both start out as the villainous rulers of isolated kingdoms, and both want to be surrounded by people who understand them, but while Ice King becomes a better person over the course of the show by learning empathy, Lemongrab gets actively worse with every appearance because he refuses to tolerate any point of view besides his own.
Flynn is this to Fionna. They are both heroic adventurers and fight off an evil wizard that has plans for their land. But while Fionna is an energetic Determinator who isnt afraid to speak her mind, Flynn is a stoic and extremely fancy Flat Character who has yet to get a speaking role or even a Day in the Limelight.
Jacques The Raccoon is this to Cake. They are both animal sidekicks to their human compatriot and help them with anything they come across. But while Cake is a sassy cat who isn't afraid to speak her mind as much as Fionna does if not more so, Jacques is a stoic and extremely fancy Flat Character who has yet to get a speaking role or even a Day in the Limelight and doesn't even show off his shapeshifting powers.
Ice President is this to the Ice Queen. They are both power-hungry Big Bad Big Bads who are even more cunning than the ice King ever will. But while the Ice Queen is The Sociopath, the Ice President is more of an Anti-Villain and appears to be like a third to Flynn and Jacques' partnership.
Lemonhope and Finn. They're both the same age more or less, and they're both seen as champions for their kingdom. But while Finn happily lives up to the challenge, Lemonhope is like a lot of kids in that he wants to be on his own and only did the right thing once because he felt guilty.
Breezy and Princess Bubblegum. Breezy, being a queen bee who loves Finn and needs a knight, contrasts PB being eternally a princess with no plans to marry and become a queen, and not returning Finn's affection. Breezy is literally everything Finn ever wanted from PB but cannot receive, and dialing up the irony is the fact that this time, it's Finn who doesn't return the affection. Lampshaded via symbolism when Finn sees her silhouette near the end of her episode, and he mistakes her for Bubblegum for a moment.
Minerva Campbell to Finn. They enjoy their work in helping others who are in need, which both can do in extreme methods. Finn takes his method of physical helping violently so long as evil is the only one trampled across his path, Minerva helps people exceptionally but peered close enough to doom her people nearly twice in her choices.
Minerva Campbell to Princess Bubblegum. They both shown their work to Finn on how they operate their respective colonies scientifically. Bubblegum is able to let go of her totalitarian approach with Marceline's help, while Minerva is still plagued by the thought of her husband abandoning her along with her own child only for said child help her in getting a hold of herself.
Minerva Campbell to Martin Mertens. Martin was a self serving sociopath who only ever cared about himself, while Minerva is empathetic to the point that she made her entire existence about helping others. Neither of them are shown to be well adjusted individuals though for very different reasons.
Phlannel Boxingday to Princess Bubblegum. She raises Lemonhope under the strict belief that he is required to free the lemon people and take over as ruler while Phlannel encourages him to do what he wants and live as a free spirit.
Peppermint Butler the Dark Magical Dude to Princess Bubblegum's Straw VulcanMad Scientist, to the point he has to pretend the crystals in his "Poison Room" are scientific not mystical.
Martin Mertens and Finn are opposite in personality and even wardrobe. They also have differing philosophies as of the season six finale. Finn grows to believe everything has a purpose while Martin sees the universe as having no purpose (it's unknown if this leans more toward Straw Nihilist or Anti-Nihilist, but given his cheery state and his neutral stance, most likely the latter. Flashbacks in the Islands miniseries furthers demonstrate their differences. Finn has his moments of selfishness while ultimately remaining a good person, while Martin demonstrates moments of heroism before relapsing into his typical It's All About Me behavior.
