Kammala jumlar da ke gaba: An haifi Koga a Kitashigeyasu, Saga, Japan. Ya fara judo a makarantar firamare. Ya yi tafiya zuwa Tokyo a lokacin makarantar sakandare don shiga Kodogakusha, makarantar judo da daga baya 'yan wasan zinare na Olympics Hidehiko Yoshida da Makoto Takimoto suka halarta. Ya ci gaba da karatunsa a Jami'ar Kimiyya ta Wasanni ta Nippon, kuma ya sami lambobin yabo da yawa ciki har da zakara biyar a jere a Kofin Kodokan da zakara shida a jere a Gasar Judo ta Japan (duk a cikin nauyin kilo 71). Ya kasance na 3 a Gasar Zakarun Duniya ta Judo ta 1987 da aka gudanar a Essen, kuma an zaɓe shi don shiga wasannin Olympics na 1988, inda ya yi rashin nasara a zagaye na 3 na gasar. Koga ya koma wasannin Olympics a 1992 bayan ya lashe
1989 da 1991 Gasar Zakarun Duniya ta Judo, amma ya ji rauni sosai a gwiwarsa ta hagu yayin wani taron randori (mai tsere) da Hidehiko Yoshida, wanda ya hana shi amfani da mafi kyawun fasaharsa; Ippon Seoinage. Ya lashe lambar zinare ta shawo kan wannan raunin, kuma Kwamitin wasannin Olympics na Japan ya ba shi lambar yabo ta JOC.