midoriya: He'd be a blushing mess, not looking anyone in the eye as he mumbles out "This is Y/n, my girlfriend."
bakugou: He won't even need to say anything-- the way he'd guard you and glare at anyone who'd talk to you is enough for anybody to know that you were his and vice versa.
kirishima: Kiri would be over-the-moon proud and happy to call you his girlfriend and he'd always bring it up in conversations to the point where his friends would get exasperated and say "yes we get it, you two are dating".
todoroki: Being the blunt guy he is, Todoroki would just plainly say "Y/n's my girlfriend" and then you'd be taken aback by how he could say it with such a straight face.
kaminari: This dork would reallllllyyy flaunt the fact that he ain't single no more in everyone's faces. Half the time his entire conversation with friends would be bragging about his awesome girlfriend.
sero: He loved to call you two 'middle school sweethearts' since you both met (briefly) in middle school. He loves to show you off.
shinso: He was reluctant to make your relationship public but after he noticed that when your label was 'single', you attracted too many guys for his liking and plus, it jeopardized his position as your boyfriend. So he made sure that everyone knew that you two were together.
monoma: He was overjoyed to declare you as his girlfriend. Everyone was kinda shocked to be honest, and since you were in class 1-A, he made sure to threaten all of your classmates to not try anything with you.
aizawa: The tension between you two was acknowledged by all of the staff members, so when you two made your relationship public, everyone was like, "Finally".
toshinori: Since he wanted to keep you safe, he only told Aizawa of your relationship. Aizawa heaved a sigh of relief once Toshi told him. "Now you can stop talking my ear off about her." he had said, which caused the both of you to be a bit flustered.
dabi: He's a hard pass on introductions. He didn't want to take you to his league because he doesn't want to get you wrapped up in that stuff.
keigo: He wasn't one to be subtle-- he brought a news crew into your part-time job to announce that he was dating you. Probably not the best move considering he was a pro-hero, but... it did make you feel special.
