Harry and Draco
I previously mentioned Harry and Draco, but I’m putting it here again because it’s such a perfect example of not taking it too far (like Benvolio and Tybalt) so they can each be their own character, but still fundamentally opposed to each other in a way that would never allow them to be friends without substantial character development. Harry, though normally at least a little level headed, has some serious hate in every sceny they share (or whenever he is mentioned) because Harry would never do almost anything Draco does, and Draco would also never do almost everything Harry does.
Kirby and Dedede
I'm sure we all know Kirby, the pink puffball with a penchant for punching gods, but have you ever considered how similar Kirby is to King Dedede, the round-bellied ruler of dreamland? Probably not, but they are very similar. In fact, they share everything but their motivation. Kirby likes doing nothing, while Dedede is industrious and ambitious. Somehow this always drives them to conflict though, and Kirby’s lack of motivation usually means he ignores the threat until it affects him personally and starts at a significant disadvantage.
Benvolio and Tybalt
Benvolio and Tybalt of Shakespere’s “Romeo and Juliet” are literally exact opposites in every way and constantly in conflict. I say “in conflict” instead of “fighting” because Tybalt is literally incapable of being on stage without trying to fight, and Benvolio tries to stop the fighting. This gives the fiery Tybalt someone to direct all of his anger at, and it doesn't end well. The problem with Tybalt and Benvolio is that Tybalt is an extreme hothead, while Benvolio isn’t never going to consider drawing his sword. If they were both equal levels of extreme, you’d be able to see that both mindsets end badly, but because Tybalt is super extreme and Benvolio isn’t, Tybalt’s personality is seen as wrong and Benvolio is seen as right. This takes away from the diversity of the characters and makes Shakespere a sith lord, because only sith deal in absolutes.
Dr Jekyll and Mr Hyde
Dr Jekyll and Mr Hyde are probably the most iconic set of contrasting characters that you’ve never heard of because you don’t read gothic psychological thriller sci-fi books from the 1880’s. The book mainly deals with honne and tatemae, or as we put it, the public vs. private self because (spoiler warning for a 125 year old book) Jekyll and Hyde are revealed to be the same person, despite one being a reputable doctor and the other being an unhinged cereal killer.