Guest had a quirk that could create poisons from their body with theirs or someone else's bodily fluid (any kind). But the problem was, that in order for it to actually be of use to people, it unfortunately needed a live person or animal to test it on. Because of that, everyone assumed Guest would become a villain and they always treated them like one. So when Guest got accepted into UA, per Nezu's specific request, everyone was on edge.

Everyone was in the common room, still dealing with the aftermath of whatever problem Mineta caused again. When Guest walked out of their room holding vials and beakers filled with what they thought were strange substances along with a bunch of notebooks, they assumed they were going to become lab rats.

But it wasn't them that Guest was testing their quirk on, it was Aizawa. And they had an agreement since the beginning of the year that he would be their guinea pig to help strengthen their quirk. Guest and Aizawa obviously kept it a secret from the other students cause no one wanted to deal with more accusations of being a villain in disguise.

The only people who knew about their agreement were Nezu, the rest of the staff members, Bakugo, and Shinso. And Bakugo and Shinso only know because Bakugo forgot to knock on the lab door ONE TIME and saw them sticking a needle into Aizawa's arm and almost blew up the whole place. And Shinso only knows because he could relate to their situation of being labeled a villain just because of a quirk.

Midoriya, having his observational skills be a curse and a blessing, sees Guest's hushed conversation with Aizawa and immediately panics because he knows their quirk and he naturally assumes the worst. Which is strange cause they'd thought he'd be one of the first ones to understand them like he did with Shinso. But to each their own.

Guest didn't realize Midoriya saw the interaction before they closed the door and left, but Aizawa did. And the look he gave Midoriya could have killed him if he stared hard enough.

After Guest closed the door, they started making their way to the labs in UA. And after about 20 minutes of waiting alone, Aizawa joined them. He was already wearing just a normal black T-shirt and sweatpants, his sleeve rolled up and sat down in the usual chair. It had only been a few months and he was already used to the routine they had.