Gladion first appears in Pokémon Sun and Moon. He was originally a member of the Aether Foundation together with his mother, Lusamine, and his sister, Lillie. However, over time, he noticed his mother's obsession with the Ultra Beasts and realized that she would ruin Alola if she let Ultra Beasts into the region. For this reason, he decided to leave the foundation, only telling Wicke of his departure. Before leaving, he stole one of the three Type: Null created to be a weapon to attack Ultra Beasts.
Upon leaving the Aether Foundation, Gladion had two goals: to stop his mother and to free Type: Null from the helmet that was meant to subdue it. He eventually worked as an enforcer for Team Skull but was never considered an actual member of the team.
Gladion first meets the player on Route 5 as a Team Skull member, but he does not seem to be interested in Team Skull's affairs, unlike the Grunts near him. After his second battle with the player at the Battle Royal Dome, he realizes the player might be able to help him stop Lusamine. When encountered on Route 13, he informs the player Team Skull is after Cosmog. Knowing now Team Skull is helping Lusamine get Cosmog back, he tells the player that they should make sure Cosmog is safe. When Lillie gets captured while the player is battling Guzma in Po Town, Gladion appears before the player upon their return to the Aether House and battles them to express his frustration. After getting his priorities straight, he takes the player and Hau to Aether Paradise to face Lusamine and save both her and Lillie. Gladion is also encountered on the way to the newly finished Pokémon League on Mount Lanakila. He thanks the player for stopping Lusamine and reveals that not only has he received a Z-Ring, but also managed to release his Type: Null from its restraint, evolving it into Silvally.
After the player has become the Champion of the Alola region, Gladion has taken over as acting president of the Aether Foundation while Lusamine is away. He may be spoken to at Aether Paradise's conservatory, where he will give the player a Type: Null of their own, along with all of Silvally's memory drives.
Gladion is also one of the possible challengers who may appear at the Pokémon League as the player defends their title as Champion.
|Non-player characters in the Pokémon series|
|Rivals||Blue • Silver • Brendan • May • Wally • Barry • Cheren • Bianca • Hugh • Calem • Serena • Hau • Gladion • Trace • Hop|
|Professors||Professor Oak • Professor Elm • Professor Birch • Professor Rowan • Professor Juniper • Professor Burnet • Professor Sycamore • Professor Kukui • Professor Magnolia|
|Other characters||Bill • Lanette • Brigette • Celio • Bebe • Hayley • Amanita • Looker • Sonia|
|Champions||Blue (Kanto/Johto) • Lance (Kanto/Johto) • Steven Stone (Hoenn) • Wallace (Hoenn) • Cynthia (Sinnoh) • Alder (Unova) • Iris (Unova) • Diantha (Kalos) • Trace (Kanto) • Leon (Galar)|