Buuxi weedha soo socota: Koga wuxuu ku dhashay Kitashigeyasu, Saga, Japan. Wuxuu bilaabay jimicsiga judo isagoo dugsi hoose dhiganayay. Wuxuu u safray Tokyo intii uu ku jiray dugsiga sare ee hoose si uu u galo Kodogakusha, oo ah iskuul judo ah oo markii dambe ay ka qeybgaleen billadyahanka dahabka ah ee Olombikada Hidehiko Yoshida iyo Makoto Takimoto. Wuxuu sii waday waxbarashadiisa Jaamacadda Nippon Sport Science University, wuxuuna ku guuleystay abaalmarino dhowr ah oo ay ku jiraan shan horyaal oo isku xigta oo ka tirsan Koobka Kodokan iyo lix horyaal oo isku xigta oo ka tirsan Judo All-Japan Championships (dhammaantood waxay ku jireen qaybta 71 kg). Wuxuu kaalinta 3aad ka galay Koobkii Adduunka ee Judo ee 1987 lagu qabtay Essen, waxaana loo doortay inuu ka qeyb galo Ciyaaraha Olombikada ee 1988, halkaas oo uu ku lumay wareegga 3aad ee tartanka. Koga ayaa dib ugu soo laabtay ciyaarihii Olombikada sanadkii 1992 kadib markii uu ku guuleystay
1989 iyo 1991 World Judo Championships, laakiin wuxuu dhaawac culus ka soo gaaray jilibkiisa bidix intii uu socday kulankii randori (sparring) ee ka dhanka ahaa Hidehiko Yoshida, taasoo ka hor istaagtay inuu isticmaalo farsamooyinkiisa ugu fiican; Ippon Seoinage. Wuxuu ku guuleystay billad dahab ah isagoo ka adkaaday dhaawacan, waxaana la siiyay abaalmarinta isboortiga JOC ee Guddiga Olombikada Japan.