its because everybody in this manga is pretty mentally ill, the teacher for example, definitely has problems (not like everybody in this series doesn't) the teacher wants to control, exploit, and manipulate someone that is someone in the state like Reiji. who is feeble and easy to the teacher is 100% a pedo and narcissistic which makes Reiji an easy victim, a native child who never makes decisions for himself. and for his childhood friend gen, he most likely is also fucked in the head with trauma. after all, he has seen his own dad have sex with his friend's mom, and is a victim too, and all his life he's been living like he's trapped since he has to follow his father's footsteps. he also has a weird possessiveness over Reiji because of his trauma. and as with Sakuko, as far as I can see, sakuko also has a couple of loose screws. but I understand because imagine wanting to escape to somewhere all your life and having your dreams crushed in front of you. in which along with the fact that your private life has been exposed to all your friends, family and classmates. and Nagi isn't really obsessed with Reiji, Reiji was just a fling for Nagi so they can commit lovers' suicide. Reiji didn't go through with it and Nagi just left. this is just what i think and i hope it helps you.
thank u thank u!! for ur explanation, it clears up a lot of my questions :) i’ve read a couple of mangas like this one,, n i never understood the reasoning behind the characters intentions of picking the mc
glad i could help!!! in manga like these, there isn’t really a reasoning behind what they did, it’s the wonders of psychological/thriller mangas!!
i don’t understand why the picked Reiji, i don’t see what’s special in him. i felt like they picked him due to his obedient nature, and how weak his mine is rn. can someone explain, pls!!??