Only One Pokmon Champion is Really Undefeated, and It's Not Leon

[هَـ-ђāŵĩ-ـَاوِي]

Pokmon's Leon has a reputation for being undefeated, or at least he did until he lost to Ash in the climactic round of the Masters' Eight Tournament. However, there's still one champion who has never been defeated on screen in a proper battle, and he didn't even make it to the Masters' Eight at all: Alder.
Carlyle Edmundson

Only One Pokémon Champion is Really Undefeated, and It's Not Leon

With Leon’s loss to Ash, who’s undefeated now?

screenrant.com