Chisao Oyama (大山チサオ Ōyama Chisao?) is a character from the MegaMan Battle Network series. He is Dex's young brother. He doesn't have a NetNavi. He speaks very childishly and believes that his brother is the best NetOp in the world.
He appears in ACDC Town, looking for Dex arriving from Netopia. He runs away crying after Lan defeats Dex in a Netbattle. He later goes with the Lan's class to Yoka and visits the zoo. Chisao is then kidnapped by a condor and trapped on a high voltage tower. He is saved when all the animals return to normal.
He appears shortly before the battle between Lan and Dex. He pretends to be kidnapped and makes Lan and MegaMan.EXE go all over the net looking for him . Chisao tries to make Lan late for the contest and miss his match.
He still idolizes Dex but he is a supposed genius and can operate GutsMan.EXE better than Dex and can perform the Program Advance. Chisao is tricked into believing that the curry shop run by the ex-WWW is a base for aliens and is given NoodleMan.EXE to destroy it but they convince them of their identities and all the conflicts are resolved in the end.
He later becomes a reoccurring character in Axess, oftentimes paired up with Rush and joining Lan and Maylu on a few adventures. He becomes hurt and cries when Dex gains a tough-guy attitude and rebuffs him from his "training" with the ex-WWW members, even rooting for Lan and MegaMan during a NetBattle with GutsMan because of it. Despite his supposed genius, he still likes to partake in regular "kid" stuff, such as making sand castles in sandboxes and other activities.