Spoiler
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Basically, the uke in his youth led a really lonely life and suffered from his parent’s emotional abuse and his tutor’s sexual and physical abuse. He ran into the super young version of the seme after running away from a physical altercation with his tutor. The kid seme at the time tended to his wounds and showed him kindness that no one else showed him before that left a deep impression. He was deeply touched and it gave him strength to keep going. Fast forward to college, he see’s the seme’s older bro in college and mistook him for the kid who took care of him in the past. The older bro knew the uke was in a dysfunctional relationship with another man (the tutor). What started off as pity eventually became affection. Their relationship becomes sexual because the uke saw his saviour in him and the older bro comes to like him (I think?). Then when the uke visits the family home, he realizes his FWB is actually the older bro of the kid in his memories and became elated in finding his saviour but couldn’t do anything to since the lil bro didn’t remember. So the Uke had the older bro dress up in the same clothes as when the sème was a kid, started to get closer with the lil bro when he visited the family home, and when the realized that the lil bro was spying on them having sex, he smiled bc he saw the lil bro was hard watching them. Eventually the older bro clocked in that the uke didn’t actually like him, but was using him as a rebound for his younger bro (the irony). Basically each bro thought they were only seen as the shadow of the other in the uke’s eyes. That is why the uke was so fond of the younger bro in ways that wasn’t there with the older bro.
I couldn’t understand the rest as to why the older bro committed suicide but that was the gist of it I think.