Nyorai mberi kweiyi ndima - Koga akaberekerwa muKitashigeyasu, Saga, Japan. Akatanga judo muchikoro chepuraimari. Akaenda kuTokyo panguva yejunior-chikoro chesekondari kuti apinde Kodogakusha, chikoro chejudo chakazopindwa nevakakunda menduru yegoridhe yeOlympic Hidehiko Yoshida naMakoto Takimoto. Akaenderera mberi nedzidzo yake paNippon Sport Science University, uye akabata mibairo yakati wandei kusanganisira makwikwi mashanu akateedzana paKodokan Cup uye makwikwi matanhatu akateedzana kuAll-Japan Judo Championships (ese ari muchikamu che71 kg). Akawana nzvimbo yechitatu mu1987 World Judo Championships yakaitirwa muEssen, uye akasarudzwa kuti atore chikamu mu1988 Summer Olympics, kwaakarasikirwa muchikamu chechitatu chemakwikwi. Koga akadzokera kumaOlympics muna 1992 mushure mokukunda 1989
uye 1991 World Judo Championships, asi akakuvara zvakakomba ibvi rake rekuruboshwe panguva ye randori (kurwisa) kurwisana naHidehiko Yoshida, izvo zvakamutadzisa kushandisa nzira yake yakanakisisa; Ippon Seoinage. Akakunda menduru yegoridhe kukunda kukuvara uku, uye akapiwa JOC Sports Award neJapan Olympic Committee.