Barry
Hired guard on the auction-camp raid who went through the storerooms instead, and was caught loading the cart
Barry was one of five men hired out of Haudh-Gwenn for the raid on the slaver auction camp, and one of the three who failed the party there (S1E10). While the fighting was still going on underground he slipped off, worked through the storerooms where the finer goods were kept, filled a sack with coin and got the cart loaded to leave with it.
Aldo had noticed him missing and went out after him. By the time Koa came up out of the cave the cart had been moved, coin was spilling out of the sack in the back of it, and Barry was lying tied beside the spillage and beaten past answering. He went back to Haudh-Gwenn with the prisoners.
Of the five, one had meant to turn the party in, one swore he had not and was not believed, and Barry stole. It left Koa saying he would never hire help again.