koga because when you go to the league the guards will check your badges. after the rainbowbadge guard checks it, the next guard checks the soulbadge which belongs to koga. ps. he is weaker than sabrina:)
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defeat all the other gym leaders
actually there isn't 9 gym leader there is only 8 like the other ones. Actually, the 9th gym leader in heartgold and soulsilver would be the first gym leader in Kanto after you beat the elite four the first time, you can face the kanto gym leaders in any order.
After defeating Koga, who is the fifth gym leader in the Kanto Region, just move on to the next city. You can get the gym badges out of order, so just move on to a city that you haven't defeated yet. Save Cinnabar Island for last, though, because it definitely is the toughest gym.
The gym leader is not at the gym and that is why we cannot enter the gym.
The 5th gym leader in Johto is Chuck. He is located in Cianwood City.