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Sending your own Trainer into battle.
You can build your own Trainer from the ground up, but requires some grinding and exploring to find and/or win battles some items that unlock some other customization options.
You start off with incredibly shitty base stats, but you find items along the way that not only determinate the trainer's type, but also the trainer's base stat. They all have fixed base stats, just like other Pokemon. These "weapons" also have their own Level. If you level them up, you will unlock more moves of the same type as that of the item, which the Trainer can learn at anytime, just like TMs.
Abilities are learned by giving the Trainer "armor" These are hold items, so you have to waste their hold item slot because of it, and the ability is only active while you're holding the "armor".
While the trainer is still active as a Trainer, a Pokemon that has the EXACT same type (combination) as that of the trainer has, it gets a very small stat boost. (x0.25 or even smaller than that).