You were kidnapped as a child and you were missing for a decade or more and you slowly went insane. The police, your parents, and your friends never found you. You had one Pokémon from when you were a child (any Pokémon you want) and they were the only friend you had. You felt betrayed by everyone you ever cared for and all you felt towards them was rage.

On your most recent birthday you injured your kidnapper and ran away. You ran past some of your old friends (Ash, Lillie, Kiawe, Sophocles, Mallow, Lana, Goh, Chloe, Hop, and Marnie) but they didn't recognize you as you quickly ran past them but you recognized them and you stopped a couple meters away and they were confused because they found you familiar and Ash was the first one to approach you

Ash: Um excuse me, who are you and are you okay? Mallow: walks over you look familiar for some reason