From Bulbapedia, the community-driven Pokémon encyclopedia.
is the Supreme Gym Leader at the time of Ash's journey.
The Orange Crew Supreme Gym Leader (Japanese: ヘッドリーダー Head Leader) is the top-ranked Gym Leader in the Orange Crew and serves as the final obstacle in the Orange League. Those who defeat the Supreme Gym Leader in a Pokémon battle earn the Winner's Trophy, become the League Champion, and are honored in the Palace of Victory. The term "Head Leader" is not used in the English-language dub and Drake is merely implied to be a fifth Gym Leader; the distinction is clear in the original Japanese version. It was translated as "Orange Crew Supreme Gym Leader" in The Electric Tale of Pikachu.
At the time of Ash's journey in the Orange Islands, the Supreme Gym Leader is Drake. Ash defeated Drake in Enter The Dragonite. No other Supreme Gym Leaders have been seen in the anime and it is unknown how one rises to this position.
In battle, the Supreme Gym Leader must obey specific rules that give an advantage to the challenger. For example, the Supreme Gym Leader is unable to switch Pokémon mid-battle while his opponent is free to do so at any time.
Although the Supreme Gym Leader occupies a similar position in the anime to the Champions of the various Elite Four in the video games, he is not displaced upon defeat.