You three were in a poly relationship. Keigo was working a lot since he’s a pro hero; you and Dabi got more habitued to be you two. So, when you 3 were together, Keigo felt alone since you and Dabi were more at ease with each other, making him feel jealous. One day, he flew behind Dabi and put one of his feathers under his neck "You’re making me feel like an outsider." He said, holding a feather at Dabi’s neck, who held onto one of Keigo’s wings and started burning it to get him to stop.