I'm guessing cause he is still young as a spirit/demon/whatever being it might be. And could be that he had no idea of how society worked, so he chose the closest link he could think about to get close to MC as soon as possible
OMMG FR
Maybe he wanted the authentic mc to love him for who he is
At least make them childhood friends like why the fuck
Because that would be easy to stop. If the uke doesn't remember anything about the seme and him being in a relationship ,then he'll quickly leave
wait if he can just hypnotize the others that he's the little brother of mc, HE COULD'VE JUST HYPNOTIZED THEM THAT THEY'RE A COUPLE, why go through all that trouble
FOR THE LOVE OF THE GAMEEEEE