The themes are songs that 你 hear in the game os on the soundtrack. 你 can hear Kairi's theme in kingdom hearts one when Sora and Kairi are talking about ditching
Roxas' theme. It portrays all the sadness he went through to get where he is...stuck in someone else, far far away from all that he holds dear. It's a sad melody of one boy's short life. It's Roxas.