When creating a Trainer, give them a name, appearance, a few personality traits, and a rough idea of their history, what their day-to-day looks like, and where they're from. All throughout creating your character, the most important question to consider is this: How do they relate to Pokemon? What role do they play in their life?
Then, give your character an ambition - a goal to acomplish, and a reason to adventure. This could be as simple as wanting to see more of the world and meet new Pokemon, or as grand as becoming the greatest trainer of all time or catching 'em all.
Many players have enjoyed playing self-inserts, or fictionalised versions of themselves. These characters should be intergrated into the world, with personalities, histories and origins like any other, and an ambition. If you think you'd enjoy a character like this, talk about it with your group during character creation - it's best that everyone is on the same page!
Mechanically, Trainers have 4 key parts to them: their Stats (Mind, Body, and Heart), Trainer Class, Special Moves, and Items.
1. First, decide you Trainer Class. This represents the kind of trainer you are, and how you relate to your pokemon, and how people percieve you. Pick one from this list, or create your own! This is largely just a title, and whenever you level up, you can choose to change your Trainer Class.
2.Then, choose a Special Move for your character. This could be from the examples here, or one you create with your GM. Moves are sorted by Trainer Class, but you don't necessarily have to choose one from your own class, and there are general moves that could suit lots of different types of characters. Pick whatever seems fun and makes the most sense for your character!
3. Next, assign their Stats. Characters have 3 stats: Mind, Body, and Heart. Chose one to have 2d (2 dice) in, one to have 1d (1 dice) in, and one to have 0d in (roll 2 dice, and take the lowest). What each stat does is explained here. Make sure to think about what stats are used in your moves when deciding!
4. Finally, give your character some starting items. The pages with example starting items isn't finished yet, so chose 1 or 2 unique items relevant to your character; 1 or 2 basic supplies like camping gear, potions, and berries; and take 10 Pokeballs.