Koga, known by the nickname Furu-Koga (distinguishing him from another Tatenokai member named Masayoshi Koga who was in turn nicknamed Chibi-Koga, 小賀), was a skilled practitioner of kendo (swordsmanship). It was originally planned that Mishima would be decapitated by Masakatsu Morita, the Tatenokai's student leader; however, Morita was not trained in the sword and failed, at which point Koga stepped in to complete it. Koga then decapitated Morita as part of Morita's own seppuku.[1]
Koga and two other participating Tatenokai members (Masayoshi Koga and Masahiro Ogawa) went on trial on March 24, 1971,[2] facing charges of bodily injury, violence, illegal possession of firearms and swords,[3] and assisting a suicide.[4] They were convicted and sentenced to four years' penal servitude, and were released in 1974, a few months early.[5]
As of 2005, it was believed that he was a practising Shinto priest at a shrine on Shikoku.[6] However, an alternative belief is that he never became a Shinto priest, instead becoming the head of the Hokkaido branch of Seicho no Ie and was renamed Hiroyasu Arechi. It is further posited that he now resides in Kumamoto.[7]