Harashima originally debuted as Hero! in 2001 and twice won the KO-D Tag Team Championship with Kudo. He changed his name to Darkside Hero! in 2005 and won the KO-D Tag Team Championship with Toru Owashi. In 2006 he became Harashima, and beat Toru Owashi for the KO-D Openweight Championship on December 29, 2006. He first defended the Championship versus Daichi Kakimoto on January 28, 2007. He beat Kakimoto with his Somato finisher. His next title defence was on February 25, 2007, where he beat Danshoku Dieno. He then defended the KO-D Openweight Championship on April 1, 2007, beating Sanshiro Takagi. Harashima lost the KO-D Openweight Championship to Koo. In a rematch on October 21, 2007 he recovered the title, becoming a 2-time KO-D Openweight Champion. He defended the title 2 times: against Mikami on January 27, 2008 and against Yasu Urano on March 9, 2008. He lost the belt to Dick Togo in a 5-way match on May 6, 2008.
Harashima and Toru Owashi won the KO-D Tag Team Championship on October 26, 2008 by beating Mikami and Tanomusaku Toba. They successfully defended it against Sanshiro Takagi and Shuji Ishikawa. Harashima and Owashi lost the championship to Kota Ibushi and Kenny Omega on January 24, 2009. Harashima got a victory in the #1 contender battle royal to face Sanshiro Takagi for the KO-D Openweight Championship. Harashima won the KO-D Openweight Championship by beating Takagi with his Somato move, giving him his third KO-D Championship. On August 24, 2009, Harashima lost the KO-D Openweight Title to Kota Ibushi in the main event of Dramatic Dream Team's event in Sumo Hall when Ibushi debuted his new Phoenix-Plex finisher. One year later, on July 25, 2010, he would defeat Daisuke Sekimoto to win the belt a fourth time. Harashima's old Hero! gimmick was later given to Minoru Fujita and on June 24, 2011, the two defeated Kenny Omega and Michael Nakazawa to win the KO-D Tag Team Championship.
- Finishing moves
- Signature moves
Championships and accomplishments
- DDT Extreme Division Championship (1 time)
- DDT Jiyugaoka Six-Person Tag Team Championship (2 times) – with Sanshiro Takagi and Etsuko Mita (1) and Muscle Sakai and Yusuke Inokuma (1)
- Ironman Heavymetalweight Championship (3 times)
- KO-D Openweight Championship (6 times, current)
- KO-D Tag Team Championship (6 times) – with Kudo (2), Toru Owashi (2), Hero! (1) and Yasu Urano (1)
- UWA World Trios Championship (1 time, current) – with Toru Owashi and Yukihiro Abe
- Hard Hit Grappling Tournament (2013)
- King of DDT (2013)
- Right to Challenge Anytime, Anywhere Contract (2013)
- 3 Organization Strongest Determination Tournament (2013)
- PWI ranked him #168 of the 500 best singles wrestlers in the PWI 500 in 2014.