
Seems hard to give a short summary for everything, especially since I suck at short summary and the story is complex with a lot of details and informations.
At which point exactly didn't you understand?
I can give you a lonnnng summary with connecting dots and plot points from the early chapters but you'll hate me forever at how long it is lol

Because Yonghyun (black haired MC) is not sure whether Sho (blond guy) is lying or not. Sho doesn't remember about the night even more he thinks they got home early and nobody found out. He received a postcard of Itami's when he's supposed to be dead so It's impossible.
3 possibilities for younghyun :
He's gone crazy and imagined everything
Sho is also being manipulated
Or Sho and midori might be in this together.
When he received the photograph, the mailman told him it was a light haired Japanese wearing a haori (just like Sho in fact) it's too much of a coincidence so he thinks he's related to Sho somehow.
Yonghyun was always wary of Sho ever since that night but this event made him almost sure it wasn't just his imagination so that maybe Sho was really lying to him, so he decided to retrieve the postcard of Itami to be sure.
So Yonghyun is angry at Sho for lying to him but also at himself because he still loves him after all that. His pride has been hurt after enduring everything so until Sho keep on lying to Yonghyun he can never bring himself to truly love him again.
And he also believe Sho had no choice but to like him because he only knew him, he had no other one to lean on.

Oh and for why he doesn't leave the mansion I think it has to do with midori being his patron. (he's being called young master so he's basically like Sho)
I also think it's because he doesn't want to leave Sho alone and because his father manipulated him so I guess he didn't want to be manipulated by his him anymore and was too ashamed to face his mother.
It's my guess for this.
Art is nice ,but I can't understand a shit