who is the brother of usui-kun ?

His name is Gerard Walker. He is the half older brother of Usui Takumi ( through their mother), since this is true that makes gerrard 1/4 japanese, but despite the bloodline difference he and takumi have a striking resemblance regardless of their different fathers. He first appeared in chapter 50, misaki mistook him for usui due to the uncanny resemblance. He and Takumi appear to be in bad terms, perhaps due to the fact that Takumi is the illegitimate child of Gerard's mother.
Hope this helps x3