1. Brock - the Pewter City gym leader who uses Rock type pokemon. He mostly uses his geodude and onix in pokemon battles. His gym badge is the 'boulder badge.'
2. Misty - one of Cerulean City gym leaders who specializes on water pokemon. On the beginning of the pokemon series, her sisters were the ones who stand as the Cerulean City gym leaders but then on the latter parts of the series, when Ash was done competing on the Johto League, her sisters called her saying that they won free around the world tickets and they put Misty in charge of the cerulean gym. This was why she became the gym leader until her sisters come back from their trip. Her gym badge is the cascade badge.'
3. Lt. Surge - the Vermilion City gym leader who uses electric type pokemon. His trusty pokemon partner in battles is his raichu, the evolution of pikachu. His gym badge is the 'thunder badge.'
4. Erika - the Celadon City gym leader who uses grass type pokemon in battles. Her most powerful pokemon is her gloom whom she raised back then when she was still little. Her gym badge is the 'rainbow badge.'
5. Koga/Janine - the Fuchsia City gym leaders who specialize on using poison type pokemon. Koga was the former gym leader but then he became a member of the pokemon league so he passed the place of gym leader to his daughter, Janine. Their gym badge is the 'soul badge.'
6. Sabrina - the Saffron City gym leader who uses psychic pokemon. She inherited her psychic powers from her father, but later then found out that her power was so much powerful compared to her father thus as her power increased, she grew more obsessed with developing it. She became less and less emotional, using her abilities for destructive purposes, even transforming her mother into a doll. This changed when Ash challenged her to a pokemon battle for his badge, Ash's befriended haunter delighted Sabrina so much, that she was no longer able to contain her emotions and started to laugh. This caused her two personalities to merge back into one. Her gym badge is the 'marsh badge.'
7. Blaine - the Cinnabar Island gym leader who uses fire type pokemon. His strongest pokemon is his magmar. At first, the His gym was at Cinnabar Island but then it got crowded with tourists not serious trainers so he got fed up and moved his gym at Seafoam Islands. His gym badge is the 'volcano badge.'
8. Giovanni - the Viridian City gym leader who uses ground type pokemon. At the series, since Gary was the first one who challenged Giovanni for the earth badge, he got to see and face of with mewtwo. He ended up loosing for the first time in a gym battle. Although, when Ash challenged this gym, Giovanni (leader of team rocket), assigned Jessie, James and Meowth to be the gym leaders thus they ended getting beat up by Ash. His gym badge is the 'earth badge.'