Izaki was always confident in school who had to work at being the best. It's very much a you get what you put in scenario. He's more than capable of rising to the task whether it's club or tennis. He also enjoys being contrary to others and seeing through their bullshit and bluster which is so refreshing. He doesn't even do it to show off, just because people's shallowness annoy him. Hakamada is the quiet one. Still waters run deep asiding from the fact he always seems to look vapid. He's the prodigy insomuch as things just come easily to him. He doesn't have to work or study and just manages to come out on top which is frustrating for Izaki who realizes he'll never reach that level no matter how hard he tries. So there's a bit of a rivalry between them but not so much because Izaki knows he's got no chance of catching up. Although, he doesn't realize that Hakamada was never competing with him. And then there's Hakamada's attraction which Izaki can't understand at all.