But. No. Actual plotting comment. So Kei came to live in the city in his teens, he fixes some of his personality but not all of it in the move from Japan (cause he was a heinous asshole sociopath back in Japan). Anyway! Since they are around the same age, they could have been classmates and find Kongo's chill radical to be around. And the two are friends over knowing each other for so long. Kei accepts he is empty and while people may think Kongo is the same as him, there is a kindness in him that calls to other people (which is why Kei didn't wake up to being a shitlord obsessed about someone potentially like him). So old classmates to friends that hang out every so often.