The masochist in him is going crazy 🤪 in this chapter. He has difficulty to feel emotions (he still has but is really low) and the only thing that he could feel was the pain (it was a big stimulation for him).
The boy IS a natural psycho, a very functional one sure, but still a TOTAL psycho.
Does anyone here remember or know Kimbley from FMA? Well, he is like him, a psychopath who understands how to navigate society and its norms, I do think Kirishima has some level of "empathy", but it is buried deep.
Yoshino's non-standard character and will picked his interest first and her personality must have bored into his "empathy" after some time, that is why he does "love" her now.
But you know... I did find it funny how easily he beat those bullies.
Google is giving us nothing. It could just be that he's mentioning the 300 days because it took Kodaka and his buddies that long to finally give him the kind of beating he was hoping for-- one that kept him out of school for about a week. Or it could be a reference to the movie 300.
Or, this might be a massive stretch, but Japan is (used to be?) an agrarian society and the kanji for three is or "三” (mitsu)-- an alternative meaning of 'mitsu' is 満つ which comes out to 'a good harvest'. So the number three has always had some kind of positive meaning in that regard.
And along those lines, it could be a 'reap what you sow' situation where for 300 (三百/さんびゃく/ san byaku) days the bullies sowed a lot of violence and mayhem, then on the 300th day they got what was coming to them. While 'reap what you sow' is a phrase specific to Xnty, it would make sense if Japanese included a similar phrase since, again, farming culture. The Miyama family's holdilngs included farm land, according to Kirishima in chapter 28.2.
And there you go. (Can you tell we've seen all the My Big Fat Greek Wedding movies? )
I still think that there're some reason for Kiri to be like this. He said it's been 300 days, how couldn't his parents notice the bruises or blood stain on the clothes and whats going on with their son?? It's been almost a year. That's neglect. And we also see that Kiri usually has dinner alone.