Bcs natsuki is older ig. I've seen it a lot, even when they're lovers if the other is older the younger would call their first name+san. But usually with natsuki and komano case, they don't need to use san cause they're only one year apart. So idk..
I think it might've also been because he was with his mother, so he still wanted to look like he was being respectful to his senpai, therefore the -san. I'm not sure ichinoshin adds it in any other context as he has cute nicknames for natsuki that end with -chan even from chapter 1. And natsuki calls ichinoshin by his family name, I think he starts with the abbreviation of his name about a year or so as they start dating ?
sorry for the ramble this is my nth time rereading and I'm finally reading the comments lolllll
genuine question— why does komano still add the “-san” at the end of natsukis name ? san is formal… also why don’t they call each other by their first names cause natsuki seemed to just found out his first name, not to mention theyre lovers and close enough to call each other by first name. sorry im just a bit confused