Ishimatsu was a yojimbo (bodyguard) that used to be part of the Kawara Gang. He left the Kawara Gang because he thought they were very weak. Ishimatsu joined the Nagatomi gang that was led by Rikiei Nagatomi, and was so skilled that he became Rikiei's lieutenant. He made his initial appearance in the third episode Hellhounds for Hire (Part 1) and his final appearance in Hellhounds for Hire (Part 2)
Ishimatsu appears in the third episode Hellhounds for Hire (Part 1) when he meets Mugen for the first time. Both of them spar, with both of them acknowledging their skills and strength, although Mugen was still very brash and cocky. Rikiei calls off the fight, and Ishimatsu doesn't appear much after this.
In the fourth episode Hellhounds for Hire (Part 2) Ishimatsu tries to lecture to Mugen about acknowledging other people, but Mugen doesn't listen. Ishimatsu accompanies Rikiei during the gamble area against the Kawara gang where he meets Jin for the first time and sees his old master Heitaro Kawara again. Rikiei tries to sabotage the gamble by saying he disposed Heitaro's dice roller, but Fuu arrives in the nick of time to be Heitaro's dice roller.
During the gamble, Heitaro wins, and Heitaro commits seppuku as a result of their agreement. Ishimatsu is shocked when Rikiei chokes Sousuke Kawara's throat while threatening to take over Kawara's territory. Mugen arrives, and in the chaos Rikiei tries to escape but is killed by Ishimatsu, who realizes that Rikiei only cared about himself and never cared for Ishimatsu, as well as avenging his old master's death.
Ishimatsu apologizes to everyone, and meets with Mugen one last time. In their duel, Mugen cuts down Ishimatsu, and Ishimatsu dies smiling.
Ishimatsu seems to be a proud warrior who originally thought that only he could be in control of his destiny. However, after noticing how the Kawara Gang became soft, he defected to the Nagatomi Gang. While under Rikiei, he did not seem to mind being under Rikiei's control until he met Mugen, where they developed a sort of rivalry.
After Ishimatsu sees Rikiei Nagatomi not following the honorable Yakuza rules, Ishimatsu kills Rikiei (part of it was for revenge for making his old master kill himself). Ishimatsu declines Sosuke to be his bodyguard, and wishes to atone his mistakes. Even though Ishimatsu is a traitor, it seems that Ishimatsu was an honorable person who is looking for a worthy opponent.