Sensei Yamada is a teacher at Niten Ichi Ryu. He is an old man, but with a mind (and later revealed, a body) as nimble and sharp as a young man, or maybe even more so. He often speaks in riddles, something which infuriates Jack, and gives the students at the school koans, which are like riddles to ponder in order to learn to meditate.
He is also a former sohei (warrior monk).
In the first book Sensei Yamada is introduced in one of the samurai classes and shows his enjoyment of giving out koans, riddles which the students must ponder and meditate on. Yori quickly becomes his star student, showing a talent for thinking them over.
Later on Yamada reveals to Jack that he is a sohei, or warrior monk, a class which was though to have died out a long time ago. He demonstrates to Jack a move called the cho-geri, or butterfly kick, which Jack later uses to defeat his opponent Raiden during a Taijutsu fight between the Niten Ichi Ryu and the Yagyu Ryu.