• English / United States: Gladion
• Japanese / Japan: グラジオ
• Japanese (Romanized) / Japan: Gladio
• Japanese (TL) / Japan: Gladio

Gladion is Lillie's brother. He made his anime debut when Ash saw a picture of him in Lillie's room. Lillie said that at that time he was "elsewhere", being a bit ambiguous as to his whereabouts. Hobbes believes he is a very kind person at heart.

A rumor of a really strong Pokémon Trainer that had been visiting Melemele Island and was accompanied by a Midnight Form Lugarugan, spread to the Pokémon School. It ended up being Lillie's brother. Ash Ketchum challenged him to a battle and after realizing that he had a Z-Ring and had battled against Kapu-Kokeko multiple times, he decided he would think about battling him. He later sent a letter to Ash to battle and they met at dawn on the beach and started their battle but it was interrupted by Rocket Gang.

Gladion left the mansion house on Melemele Island about half a year before Ash Ketchum arrived. According to Lillie and Hobbes, all of a sudden, he said he wished to study by himself and then he wished to try giving his life and his future some considerable thought, all while polishing his Pokémon battling skills. When she saw him again, she noted that he seemed like an entirely different person from the kind brother she once knew.

Lillie loves Pokémon but is unable to touch them because she was attacked by an Ultra Beast. Gladion was so terrified he wasn't able to save her but Type: Null saved them. Lilie has never mentioned anything about this to Ash Ketchum or her friends because she can't remember it as that experience frightened her that badly. Gladion told Ash Ketchum the story because he trusted him but he's vowed to protect her so that she'll never experience anything that frighting ever again by becoming a Pokémon Trainer and become stronger together with his Pokémon for the purpose of defeating every single Ultra Beast there is. Because of that experience, he believes they are creatures that bring misfortune to mankind and that they should not exist in this world, including Nebby.