To clear up confusion, here is every trainer confirmed to be in so far.

There are 65 Trainers available at launch, with more to come in updates.

Trainers with Confirmed Sync Partners:
>Male Protagonist with Pikachu
>Female Protagonist with Pikachu
>Red (New Design) with Charizard (evolves into Mega Charizard X)
>Blue (HGSS) with Pidgeot
>Brock (HGSS) with Onix
>Misty (LGPE) with Starmie
>Erika (FRLG) with Vileplume
>Lance (HGSS) with Dragonite
>Clair (HGSS) with Kingdra
>Brendan (ORAS) with Treecko
>Norman (ORAS) with Slaking
>Flannery (ORAS) with Torkoal
>Steven (ORAS) with Metagross
>Barry (DP) with Piplup
>Crasher Wake (DPPt) with Floatzel
>Cynthia (DPPt) with Garachomp
>Thorton (Pt) with Brozong
>Cheren (B2W2) with Stoutland
>Elesa (B2W2) with Zebstrika
>Skyla (BWB2W2) with Swanna
>Iris (B2W2) with Haxourus
>Caitlin (BWB2W2) with Reuniclus
>Grimsley (BWB2W2) with Liepard
>Rosa (B2W2) with Snivy (Was shown with Serperior in promo art, maybe Pokemon evolve over time in-game?)
>Korrina (XY) with Lucario (evolves into Mega Lucario)
>Wulfric (XY) with Avallug
>Diantha (XY) with Gardevoir
>Professor Kukui (SMUSUM) with Lycanroc
>Acerola (SMUSUM) with Palossand
>Nanu (SMUSUM) with Alolan Persian

Characters with Unconfirmed Sync Partners:
>Professor Oak (FRLG)
>Sabrina (FRLG)
>Giovanni (FRLG)
>Lorelei (FRLG)
>Kris (Crystal)
>Lyra (HGSS)
>Silver (HGSS)
>Bugsy (HGSS)
>Whitney (HGSS)
>Pryce (HGSS)
>Will (HGSS)
>Archer (HGSS)
>Ariana (HGSS)
>Wally (ORAS)
>Wionna (ORAS)
>Tate and Liza (ORAS)
>Fantina (DPPt)
>Candice (DPPt)
>Flint (DPPt)
>Cheryl (DPPt)
>Palmer (DPPt)
>Hilbert (BW)
>Hilda (BW)
>Cilan (BWB2W2)
>Chili (BWB2W2)
>Cress (BWB2W2)
>Hugh (B2W2)
>Calem (XY)
>Professor Sycramore (XY)
>Viola (XY)
>Siebold (XY)
>Wikstrom (XY)
>Hau (SMUSUM)
>Lillie (SMUSUM)
>Gladion (SMUSUM)
>Lusamine (SMUSUM)
>Mina (SMUSUM)
>Sophocles (SMUSUM)
>Kahili (SMUSUM)

Minor Trainers (Unsure if playable or not...)

>FRLG Lass
>XY Swimmer
>ORAS Camper
>ORAS Poke Fan
>ORAS + SMUSUM Ace Trainers