In the first book, Sensei Kyuzo is shown to have a personal hatred of Jack for no apparent reason and makes his life a misery, picking the young english teen for all the physical demonstartions. Jack's rival Kazuki is Kyuzo's star student and he seems to take delight in watching Kazuki beat Jack in practice.
Sensei Kyuzo makes a reappearance as a doshin, using the alias Renzo. He is part of a squad that arrests Jack and Benkei on the charges of illegal gambling. Bringing him back to the bugyo, Sensei Kyuzo obeys his orders to carry out yubitsume on Jack, therby cutting off the tip of his little finger.
Later on, as Jack tries to retrieve his belongings, Sensei Kyuzo bears down on him. When Jack accuses him of lacking any respect for bushido, Sensei Kyuzo responds by telling him that post-war, he had been forced to hide for fear of reprisal, and became a doshin, serving someone who isnt even a warrior. He then says that only the capture of Jack can redeem his status as a respected samurai once more.
Disappointed and angry that his former teacher's vendetta was loss of face, he then accuses him of violating the very spirit of bushido itself, having betrayed Masamoto. Sensei Kyuzo retaliates that he owes Masamoto no allegiance, the swordmaster having surrendered, and should commit seppuku instead.
Jack retorts, saying Masamoto has not surrendered. He asks him that if defeat meant shame, he ought to commit seppuku himself first, but he had not. Sensei Kyuzo then cracks his knuckles, saying that he has an old score to settle first.
While initially gaining the upper hand in their fight, Sensei Kyuzo is caught off-guard by Jack's ninjutsu techniques, which he had not known about. He manages to fight back but their fight is interrupted by the bugyo's Akita dog. He kills the dog at the cost of it maiming his right arm, then pounces on Jack using Yoku Sankaku Jime, a triangular choke. Just as he is about to kill Jack, the latter manages to grab his kunai and stab Sensei Kyuzo in the leg. Downed, Sensei Kyuzo goads Jack to kill him, but he knocks him unconscious instead.
After Kazuki is arrested, Masamoto reveals to Jack in the post-celebrations that Sensei Kyuzo has committed seppuku out of shame of his actions, and that he sends his apologies. Jack accepts it and honors his taijutsu master for his efforts in the Battle for Osaka Castle.
Physicaly, Sensei Kyuzo is short, and doesn't appear to be much of threat, those who think so are badly mistaken and find themselves regretting their mistake. He is a dedicated Taijutsu practitioner and teacher, a master of his art. Kyuzo is also a complete master of hand-to-hand combat and can defend himself against absolutely any opponent. He can defend himself bare-handed against any and all weapons, and can turn hand-held weaponry against the owner. He possesses incredibly fast and precise reflexes, and can catch an arrow in mid-flight.