Yuu Mizuno is the main deuteragonist of 'Sanrio Boys'. He is one of the best friends of Kouta Hasegawa and Shunsuke Yoshino who has a strong liking for the Sanrio mascot My Melody, but he is not shy to openly admit his love for Sanrio.

Yuu is a very open, friendly, and kindhearted person who gets along well with almost everyone, except with his sister, Yuuri Mizuno, where they both appear to have some sort of love-hate relationship.