Hey uh

Why does Iida Tenya call Tsuyu “Tsuyu-chan-kun” does japanese do that??? Or???

-chan and -kun are both things you attach at the end of names depending on context of the relationship. -chan is used usually for girls and little kids. It has a bit of pet name kind of feeling, so it can be inappropriate or even derogatory depending on circumstance. (For example a male character referring to a female character with -chan right off the bat without permission could be seen as looking down on them, flirting in non-respectful way, infantalizing with cooing baby voice, etc.) It’s a very unusual character quirk of Tsuyu that she ASKS others to call her Tsuyu-chan, combination of her first name+chan. That’s an extremely informal and cutesy way to call her, and it’s no surprise that Midoriya repeatedly stuttered when he tried to call her that way.

-kun is usually thought as a thing for boys, and while it’s mostly true in informal conversations, it can be used for girls too but usually in formal, official setting, like teachers talking about female students or people talking about their workplace colleagues and juniors. It’s Iida’s character tick that he calls ALL his peers with -kun, boys and girls altogether. It’s viable as Japanese usage but is unusual, and nobody other than Iida does this in MHA.

Tsuyu-chan-kun is the result of Iida sticking to his usual speech habit of using -kun suffix to remain formal and professional and at the same time trying to respect Tsuyu’s wishes that she be called Tsuyu-chan.