Using his knowledge and experience as a former sergeant in Japan's Ground Self-Defense Force, he provides advice for battle action scenes, military roles, weapon design, and so on.
He served as the chief military advisor and had guest appearances in many movies, including Battle Royal (2000), Returner (2002), Battle Royal II (2003), Last Samurai (2003), Calamari Wrestler (2004), Casshern (2004), Devilman (2004), Godzilla Final Wars (2004), Ultraman (2004), Lorelei (2005), Saikano: She's the Final Weapon (2006), and Sukeban Deka which he has a major role in.
Most recently he has worked as a military advisor for parts 1 and 3 of 20th Century Boys, and he was a consultant on the, recently released to Blu-ray, Space Battleship Yamato live action movie.
Koshi is a well-known writer for the popular Japanese periodical called "Arms Magazine". He has also published articles on the history of weapons and uniforms of the Japan Self-Defense Force.
He is also a talent manager who works closely with the Japanese anime and J-pop industry. He heads up the Big Fighter Project (B.F.P.) which has been producing talented artists since 1996.