I'd totally fail a Japanese immigration test because I'd have said: "Sena-chan."
It's funny how I've spent a third of my comments calling Nao immature, but now Ayato's first words to him were about Nao looking so mature. Sure, he's already grown to his full male height, but that's about it with his maturity.
he's actually progressed pretty substantially just to this point in the series.
Yes, he's a touch tsundere-ish when it comes to how he perceives Sena even to this point, but he's just
extremely blunt and to-the-point, which is something that is really not all that common in these sorts of stories (especially for a tsundere character, who usually hides their true feelings behind their mannerisms).
And he's now actually speaking his mind and his intent, and following through with his actions to boot. The moment he actually confessed and set his mind to pursuing her, Nao's whole demeanor has matured. It's just that he still doesn't mince words and will call someone being an idiot, an idiot.