>>45756803Hm. While I don't have a template, you could start by figuring out your Trainer's personality then follow that up with where they met the Pokemon (What route) then look at their nature and likes. Think back to old battles where they stood out the most and did something memorable make that aid in developing a personality.
For example, I imagined for the entirety of my Sword playthrough that my female Sirfetch'd was out for gym leader and Bea's blood, she'd solo so many of them that at any point a battle came up with one I'd switch her in and the rest of the team was just watching with popcorn going:
https://youtu.be/U1UtRnGn5hc 