Boshu is an old man living in Shinjuku and is, until the appearance of Tetsuya, considered to be the best Mahjong player in the area. He is able to defeat Tetsuya in their first match and shows him that gambling isn't just about "testing your luck against others" but also about skill.

Boshu becomes Tetsuyas mentor after a while. His teaching style is to throw Tetsuya in at the deep end: Even Boshu teaches him some basic skills and gives small advices, but let's Tetsuya make mistakes and leaves him alone on finding a way to solve a problem. Eventually, Boshu and Tetsuya team up to become the best gambling team in Shinjuku.