Onion is the reclusive Ghost-type Gym Leader of Stow-on-Side in Galar. Characterized by extreme shyness and a preference for solitude, he avoids public interaction. To manage social anxiety, he consistently wears a mask and keeps numerous identical replacements on hand. He finds comfort in isolated locations like cemeteries and ruins.

A pivotal incident at age four granted him the claimed ability to see deceased Ghost-type Pokémon; the event's exact nature remains unspecified but is intertwined with his deep connection to these Pokémon. Ghost-types like Gastly, Haunter, Gengar, and Cursola are his frequent companions. Despite his reserved nature, he exhibits a playful, mischievous side, sometimes using his Pokémon to startle unsuspecting individuals.

Within *Pokémon: Twilight Wings*, Onion's development unfolds. In "Moonlight," encountered in a cemetery during a full moon, he initially hesitates when approached by Tommy, a boy seeking help to visit his hospitalized friend John. Onion ultimately assists Tommy in covertly entering the hospital, facilitating Tommy's reconciliation with John. Later, in "Sky," he participates in an interview discussing Champion Leon and attends Leon’s battle against Raihan alongside Tommy.

In "The Gathering of Stars," he is depicted without his mask in the cemetery, accompanied by Cursola and other Ghost-types. He subsequently participates in the Galarian Star Tournament, appearing publicly during the opening ceremony. This progression illustrates his gradual, though reluctant, integration into broader communal events while maintaining his solitary core.