Sometimes it's easy to lean into how a species of mon is normally characterized, but it's also fun to sometimes give a mon a personality most people wouldn't expect.

Honestly, their natures (and to some extent, how their stats are distributed) serve as the basis. Also, the Characteristics you'd find on their main pages (i.e. "Likes fiestas"/"is proud of its power"/"Quick to flee"/etc.), which reflects the aforementioned.

Honestly I kinda take cues from the Pokemon anime. I try to give them some sort of quirk while also considering what sort of role they'd have in the team emotionally. I also consider what their battle style is like and try to factor in what sort of temperament would lend itself to that sort of style...

Really, I just treat the Pokemon as if they are OCs themselves. They are, after all.