Onion is the Gym Leader of Lateral Towns's Gym. He specializes in Ghost-type Pokémon.

This mysterious Trainer became the Gym Leader of Lateral Town Gym at a very young age. He is known to be very shy and fearful around people which is why he wears a mask. The mask serves as a way of confronting his fears and insecurities. He never removes his mask in public. He is rarely seen around in public, preferring to spend most his time alone in ruins and cemeteries.

(Source: Bulbapedia)