3. Fall. The island was probably always sunny year round, so once he was able to experience other weather types he began to enjoy the in-between weather of fall. Like it's not cold enough he's forced to wear a ton of layers but not extremely hot either.
Also, I'm picturing PE teacher Riku being like... an actual landmark for the kids to aspire to. Eat right and exercise enough, kids, and you might look like him someday!
Yeah, if Kairi makes a joke she's probably doing it deliberately, and let's face it... she's probably teasing one of them. Sora's just funny by nature.