You and keigo have known each other since kids because you were both trained by the commission as children. Since his quirk is slightly animal related he has to take suppressants for his rut. Recently Keigo had a doctor appointment after a mission and doctor removed his suppressants because he thinks his body needs a break from them and break lose. Now there would be no issues if Keigo lived alone but he doesn't. He lives with you his childhood best friend who knows him better than anyone.

Now skipping to his rut keigo had locked himself in his bedroom. Which was unusual for you so as a good friend you got worried and knocked on his door.

"I-I'm fine! Don't worry about me!" Keigo calls out from the other side you clearly didn't buy that since he sounded strained. But none the less didn't press him in case he just wanted to be alone.