Honestly, the central pair is usually what hooks you in any rivals story, but 'Red, White & Royal Blue' is a classic example that nails the dynamic. Alex and Henry are the obvious core—the political kid and the prince, forced into a fake friendship that turns into something real. Their verbal sparring is half the fun. You've also got Nora, Alex's best friend, who's the actual genius and provides a lot of the grounding and witty commentary. And you can't forget Ellen Claremont, Alex's mom and the President, whose political career adds a huge layer of pressure. The novel really spends time making the rivalry feel personal and political at the same time, which gives all the side characters a role to play in either fueling the conflict or helping them navigate it.
Sometimes I think June, Alex's sister, gets a bit overlooked, but she offers a different kind of family insight compared to the very public figures. The cast isn't enormous, which lets you get properly invested in each relationship. The key is that everyone around the main rivals either directly complicates their situation or becomes part of their support system, making the whole 'enemies to lovers' arc feel earned rather than just a trope.