Something I tend to do is match humans up with their signature Pokémon. It’s like choosing which Pokémon they would turn into if they went to the Mystery Dungeon universe or something. However, there is one problem: sometimes more than one human has the same signature Pokémon. Below is a list of trainers with their main Pokémon as examples. Keep in mind that this is only for the games.
Maxie and Courtney both use Camerupt
Archie and Matt both use Sharpedo
Sidney and one member of the Shadow Triad both use a Bisharp
Aaron and Celosia both use Drapion
Cyrus, Zinzolin, and Mable both use Weavile
Iris and Drayden both use Haxorus
Bruno and Bea both use Machamp (though the latter’s can Gigantamax)
Agatha, Morty and Allister all use Gengar (though the last one’s can Gigantamax)
Grimsley and Bryoney both use Bisharp
Lorelei and Melony both use Lapras (though the latter’s can Gigantamax)
Sordward and Shielbert both use Sirfetch'd
Klara and Avery both use Slowpoke (for a while at least)
Rose and Peony both use Copperajah
Valerie and Penny both use Sylveon (though the latter’s can Terastallize)