Haruka: Solitary Hide-and-seek by koyori. I cannot explain why it fits him, I just know that I feel this song closely relates to him?
Touma: Teo. The lyrics are really uplifting but they have a hint of pain and despair, especially the chorus, is a song I see him singing to zool.
Minami: Misery by Yuyoyuppe. The lyrics are simply so... Minami-coded, just read them, trust me.
Torao: I have too many choices for him... But I think Datte also by Yuyoyuppe is incredibly him, so please read the lyrics wherever you have a chance!

For songs I'd like them to sing because it fits their voices...

Haruka: Tokyo Teddy Bear by neru
Touma: Shanty by WOTAKU
Minami: Crescent Moon by Kurousa
Torao: Fixer by nuyuri