Dai Matsubara is Karou/Buttercup's brother. He is also Shou Matsubara (Sam)'s brother.
In the English dub, his name is Dave.
His voice was heard in And then There Were Three, where he was getting Buttercup's things. He later appeared in All Ken Wants for Christmas (Episode 26) for the first time in person although he had no speaking role, and then again in Trading Faces (Episode 41).
He uses Boxers and he walks around his house in this, how is confirmed in te episode 41 of the serie