beta
Rather than having a crush on her, I think he just found someone who can consider as a genuine friend.
And that he just has high awareness in sensing emotions/feelings which why he figured that the girl likes his brother. (and the other first scene where he was making n estimation of "two weeks" with his friend and their gf)
The reason why he's mad was bcs the brother was just using the girl to retaliate against him. Bcs the girl likes his brother genuinely but the brother doesn't and just used her love for advantage :(