Masahiko Katou is Jeri's half brother. He actually appears only once in the show, when he was playing with Calumon. Unfortunately he fell down and cried until his mother came to comfort him. Meanwhile false Jeri, D-Reaper-agent, stood by and watched Calumon. He was hurt and confused when the false Jeri walked over to him and proceeded to pin him down to the floor with an evil look on her face stating "All suffering must be eliminated."
Masahiko actually has a good relationship with Jeri. He often plays with her and loves it when she uses her dog hand puppet.
His role in Dimensions has been small so far, though he has been mentioned a few times. Recently, he was present when his older half-sister was confronted by an angry crowd for being a tamer.