Kotetsu Boku

Boku faced Arnaud Lepont at ONE FC 14 on March 14, 2014. He won the fight via TKO in the first round.

Boku lost his ONE FC Lightweight Championship to Shinya Aoki at ONE Fighting Championship: Kings and Champions on April 5, 2013. Aoki fought to avenge his Evolve MMA teammate Moreira, and forced Boku to submit at 2:01 during round 2 with a rear-naked choke.

On September 18 it was announced that Boku had signed with the Singapore-based ONE Championship and would be fighting Zorobabel Moreira for the ONE FC Lightweight Championship at ONE Fighting Championship: Rise of Kings. Boku faced Zorobabel Moreira for the inaugural ONE FC Lightweight Championship on October 6, 2012. Boku managed to endure vicious leg kicks throughout the majority of the fight to eventually earn an upset comeback TKO over Moreira early in the third round.

2009 saw Boku return to Shooto. His first fight was against then-Shooto Lightweight Champion Yusuke Endo in a non-title fight on January 18, and ended in a draw. He then fought Yutaka Ueda, and won, before taking on Endo again in a rematch which was a title-challenge this time. This took place at Revolutionary Exchanges 2 on September 22, and Boku was submitted in the first round. He then took on Yukio Sakaguchi at Revolutionary Exchanges 3 and won by TKO in round 1.

Finland's Jarkko Latomaki was Boku's first opponent in Cage Force. They faced each other at GCM: Cage Force 2 on March 17, 2007, and Boku came away the victor after knocking Latomaki out with an elbow in the first round. He then fought at Cage Force 3, 4 and 5, winning all but one fight which he lost to Artur Oumakhanov. After this, he went into DREAM for one fight. It was a loss to Joachim Hansen at the DREAM Lightweight Grand Prix 2008 Opening Round on March 15, 2008.

Boku's K-1 HERO'S debut came against Brazil's former WEC Lightweight Champion Hermes Franca at HERO'S 3 on September 7, 2005, and he won by majority decision. His next, and last, K-1 HERO'S bout took place on August 5, 2006 at HERO'S 6 against Alexandre Franca Nogueira. He also won this, by unanimous decision. After this, Boku returned to Shooto for one fight, a draw against Kenichiro Togashi at Shooto: Back to Our Roots 1. He then joined Greatest Common Multiple in 2007.

A Zainichi Korean originally of ancestral Joseon citizenship, Boku acquired South Korean nationality in 2004.

Boku made his professional debut against Dutch fighter Marc Duncan at Shooto: GIG East 7 on November 26, 2001, and won by submission in the second round. He also won his next two fights, against Takuhito Hida and Mitsuo Matsumoto, before losing to Takaharu Murahama at Shooto: Treasure Hunt 6 on May 5, 2002. He won his next fight against Toniko Junior, but again suffered a setback by losing the following two fights against Mitsuhiro Ishida and Kenichiro Togashi. Boku then had another four fights in Shooto, winning all four, before transferring to K-1 Hero's in 2005. He was the Shooto Pacific Rim Welterweight Champion, but vacated his title when he left for K-1 Hero's.