Here is a list of who I headcanon to be the PC in each respective game. The other protagonist will take the role they have in game unless they don't have a role in game. I will share my headcanons for them, too.

FR/LG: Red
HGSS: Ethan
ORAS: May
DP/P: Dawn
BW: Hilda
BW2: Rosa
XY: Serena
SuMo: Moon

Leaf was the last trainer from Pallet but she never made it big

Kris worked as Professor Elm's in the field assistant

Sun was Moon's older twin brother who didn't get far into the island challenge before giving up.

I know most of my headcanons PCs are female but it just turned out that way...