Kirishima never saw himself as a jealous guy, it was never his style! He was always smiling or cracking jokes, portraying himself as the boisterous and lively person that he was. Known for his kindness, he always sought to uplift everyone's spirits and pump them up.
He was never jealous of any guys coming to talk to you of course he trusted you, you were his girlfriend it was only natural that you’d had a few guys come to talk to you.
He insisted that he wasn't feeling jealous, but when he noticed Bakugou gazing at you as if you were the most alluring person in the room, it stirred something within him. He kept saying that it wasn't jealousy, but he couldn't help but feel a twinge every time Bakugou chose you as his training partner or assigned partner for a task given by Aizawa.
Kirishima hoped that it wasn’t jealousy he felt but whenever he was beside you, Bakugou would always ignore him to talk to you giving you all his attention, but if it was only him and Bakugou they were able to have a conversation as if things were normal. He even took noticed that during training Bakugou would always try to show off, by removing his shirt or completing a difficult move.
It gotten to the point where Kirishima would always be stuck by your side more often, butting in on the conversations Bakugou had with you. Always insisting on being your partner either in training or in-class projects, giving Bakugou no time to come up to you or ask to be his partner. It wasn’t like he was insecure, he just didn’t know what his best friend was trying to do with you, did he.. like you?