Kei's(nii chan's) Childhood is being messed up by an adult guy *too*(ojisan). So since kei experienced that kind of things when he's still a kid, he make a revenged by doing it to Yui. When Kei's realize that he just did what happened to him when his a kid he feel sorry/guilt to Yui, so he's looking for Yui again to love him or accept him(like I mean he correct his mistakes)
SPOILERRRR ALERT!!
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KNOX [blonde seme] already found out what they're doing and regretting his action for turning down the uke[bottom]. Nowwww expect threeeeesomeeee!