Tsuyu is the UA traitor and Mineta finds out.
Tsuyu left her phone on the table in the main area of their dorm, her attention called away by Ochako. Everyone else was occupied leaving only Mineta alone in the lobby and sure enough he spotted Tsu's phone.
Looking around to make sure no one was watching he leaned over and picked it up to start digging through it hoping to learn something he shouldn't about a girl he secretly really really likes.
After a moment of browsing (Assume Tsuyu either didn't put a password on her phone or it was ribbit okay that part isn't important) Mineta noticed a text notification from an unknown number and began to delve into Tsuyu's texts. Thankfully he isn't so messed up he'd get involved with her family so he just stayed to this specific number because he didn't recognize it and what he found made his blood run cold.
A back and forth between an individual asking for information on classmates, teachers, and locations at UA with some vague threats sprinkled throughout. Mineta couldn't believe it, it means everything that happened at The Beast's Forest, the USJ, the mall...Tsuyu knew it was going to happen and she helped them.
Mineta attempted to take a deep breath, Tsuyu wasn't a villain, she was being targeted but why was she still willing to help them? AFO was locked up so she'd be safe- and before he could think of anything he heard a group returning to the lobby. Panicking. He turned off Tsu's phone and put it back on the table gently, he then got up and made his way towards the kitchen as if nothing happened.
As everyone funnels back through the lobby once more, Mineta looks over and sees Tsuyu grab her phone, not even suspecting anyone went through it. He lets out a sigh, but now he has to tell somebody and soon. A problem then occurs to him, he only found out because he was snooping through her phone so he can't tell anyone else, and without any direct proof he wouldn't be able to convince Aizawa, and he couldn't bring himself to make Tsuyu confess it to everyone.
Mineta realized his inner mumbling made him sound like Izuku...wait IZUKU! He's a lot more level headed-ish and is friends with Asui so surely he'd be easier to approach on this, but it also means confessing to snooping through another student's personal items which would land him in trouble, but if Izuku is made aware it'd be worth the risk.
So he'll do it, for Tsuyu.
(Sorry if this is kinda dull I'm not the best at fanfics but it was on my mind)