1/3 nlh: KK oop facing turn all in (one player already all in on flop)

1/3 NLH match the stack

This hand was from a few weeks ago.

Main villain is the button in this hand. He is a reg who plays well, when he is having a winning session and tends to become the whale when he’s having a losing session. This session, he was winning. Either way, plays a bit more aggro than the avg player at the table.

The limper in the hand is just a fishy kid who plays 1/2 usually and I never really seen him play a match the stack format.

1400 effective
Preflop:
UTG straddle 6. Co limps. Btn raises to 25. Sb folds. I have black KK in the big blind and 3! To 100. The limper in CO calls 100. Btn now 4! To 400. I flat. Limper calls.

(Pot: $1200) Flop Q65ssx.
I check. Limper check. Btn 400. We Call. Limper is all in for less (375)

(Main Pot: $2,325) (side pot:$50) Turn 3x.
I check. Btn is all in for our remaining 600.

(Side pot: $650)
(Total Pot: $2,975)

Hero?