Tbh I think it’s just the fact that he could dig deeper into him and find out the truth that he’s basically a male, not a female like how he introduced to his family
Minjoon was a foreign student in Tokyo University right? So basically that is the only connection/information he has in regards with his background. So if they confirm it that he indeed is from Tokyo U. Then they can connect the dots and wil lead to how Minjoon was deceiving the Ueyama elders .And might involved him on something dangerous.
bcs minjoon registered in tokyo univ using his real name and gender. and daichi lied to the family that minjoon is a woman named yuri who is his girlfriend and taking care of toma. if u remember, minjoon was baited to go to tokyo univ to get his stuff and he got kidnapped there so at that time, the brother already guessed minjoon wasnt a woman.
But he can get to know he's male just by looking and talking to him?
Can someone remind me again why is it a big deal that he knows that minjoon is studying at tokyo univ?