Millie and Christopher Chant from Conrad's Fate, by Diana Wynne Jones.
So in my fanon, Christopher is very upfront about being heads over heels for Millie (he's only half-in love with her in Conrad's Fate already) and Millie has no patience for it because, augh, who would have the patience for Christopher! And rejects him. And he spends a good long time driving everyone else insane with his beyond-his-years epiphanies, whether they want to listen to him or not, about the woes of unrequited love, which leads to him having manly bonding discussions with Mordecai on the nature of how much love sucks for men over cricket. And Millie spends her time giving frustrated huffs over it with Conrad, who is also getting it from Christopher, and Conrad's just "look I don't blame you but can you two please stop talking to me about this I mean OH MY GOD."
Basically I want this romantic farce to happen.