I try to give further information about my problem. I apologize for any grammar errors. English is not my first language.
I didn't create this Sim, I just gave a townie a make over in order for my sim to date him. (Legacy Challenge) She had not met him before, but when I met him in Mt. Komorebi, he was in that stupid Yama-chan mascot suit, so I couldn't interact with him. I didn't know the game has decided him to be Yama-chan before. I added him to the household so he can build a relationship with my sim but the bars won't go up. All the normal stranger interaction options are available but they don't know each other. I tried to reset the corrupted sim several times, tried every option in the MC-Command Center, restarted the game, tried to repair it but no changes at all. They still can't get to know each other. I even aged him up and down - nothing. The only thing I haven't tried is to create an identical looking twin as I did not wanted to redo all the outfits but it seems like I have to.
Edit: The "identical twin" solution worked
