When I say "major characters," I'm talking about trainers such as Cynthia, Cheren, Bianca, N, your rival, etc.
This does not include trainers in PWT.
Floccesy Town - Alder
Undella Town - Cynthia
Striaton City - Cilan, Chili &Cress
Black City/White Forest - Benga
Plasma Frigate - Colress
Nuvema Town - Bianca
Route 5 - Cheren
Castelia City - GameFreak
N's Castle - N (This varies in the season) (Long after this, after visiting him at the Ferris Wheel in Nimbasa on Fridays, you will be able to rechallenge N as he wonders about the previous game's Trainer and if he achieved his dream).
Source - Serebii
Source 2 - N
Hope this helps and what you looking for!