Preflop
Hero UTG opens to $1.50 125bb w/ KK
MP folds
Co folds
Bu (fish) calls
sb folds
BB (reg) squeezes to $7 125bb

Hero 4bets to $16.5
BB (reg) calls

Flop

As Ac 5d pot $33.5

Hero bets $8.5
BB (reg) calls

Turn 6h pot $50.5
Hero bets $17.5
Villain calls

pot $84
Villian donks $25 all in
Hero ???

I know river's never a call vs a reg. Pretty much 99.99% percent of regs will never donk jam weaker right, even KK? Regs likely a weaker reg due to donk jam. Believe there is some history and with reads that he is an overly aggro reg but only about 80% sure those reads match the SN.
And how bad is the turn 1/3rd? I mistakenly thought for some reason the pot would be left with about half pot size bet.

Also would 1/3rd size be a size that regs overfolds too such as a lot of pocket pairs KK-99 in a pot that leaves a half-pot size bet-2/3rd on river ? Making river bluffs likely to get called ? or the opposite?