• English / United States: Allister
• Japanese / Japan: オニオン
• Japanese (Romanized) / Japan: Onion
• Japanese (TL) / Japan: Onion

Allister is the Gym Leader of Stow-on-Side's Gym in the Galar region. He is a mysterious boy that has an emotional bond with Ghost Pokémon and wears a mask.

Ash Ketchum and Goh went to Shoot City Station to catch a train to Lateral Town. After seeing a masked boy drop a ticket, they decide to get on the somewhat creepy-looking "Ghost Train" in order to return it. That boy turned out to be Allister.

Ash Ketchum and Goh then went to visit Allister in Lateral Town, in order to make Gangar Kyodaimax. Allister tells them that Kyodaimaxing Ash's Gengar required having it drink Max Soup made from Max Mushrooms and that to gather the Max Mushrooms, they'd have to get them from a forest where there was a lots of unique Ghost type Pokémon.