This article contains major plot spoilers from the manga that have not been revealed in the anime. Proceed at your own risk.
Childhood friend and former childhood sweetheart, before Koyuki moved out after their parents' divorce they used to spend a lot of time together. Being fans of The Hedgehogs, Yuu was known as Nico, while Koyuki was Tama.
Fuuka is Yuu's late girlfriend and former bandmate. After Fuuka died, Yuu was struck with a sense of grief that lead to him replanting about his love live and the band.
After Akitsuki died, Yuu met Fuuka Aoi sometime after and she joined Blue Wells, along with becoming his girlfriend.
Friend and principal counselor during his relationship with Akitsuki, Yuu used to refer to him as the typical gay.
The character that Yuu at first confused with a "bad boy" of the school, although later he realizes that he is not a bad person. He also becomes the leader of the band "the fallen moon" and for a time they used his garage to do the band's practices
Friend of Twitter and later partner of the band, in some occasions it has been shown that Sara has a romantic interest for Yuu.