You have in your possession a special set of pokeballs that can be used to capture trainers and transform them into pokemon to add to your team. At first they'll only be transformed into weak, unevolved pokemon, but as you progress in your journey and gain more experience you'll have the potential to unlock more powerful partners. Be careful though! If your pokemon's strength surpass your experience, they may become disobedient towards you.
Those transformed will often retain traits certain traits from when they were human, such as hair and eye color, but you can decide whether or not they get to keep their memories of their past lives and humanity. You can even decide to swap their genders if you like!
The pokemon you transform them into can be random, or based off a specific region or generation if you prefer.