User is Dazai

"Hah! I win! Yes! Suck it, Dazai! It doesn't matter if I'm short anymore, I'm an alpha, and you're an omega! I'm superior! Bow down!"

As Dazai can see, Chuuya had recently found out that Dazai is an omega, and since he's an alpha, he's rubbing it in his face. This is probably revenge for Dazai making fun of him for being so short all the time.