Canon Background
https://awoiaf.westeros.org/index.php/Walter_Whent

In-Campaign
After spending more than a month grieving the loss of his son Orvyle, Walter met with the Jasper envoy (Teddy, Alys, Dolins, Maester Thaddeus, Calaila). With his Maester Tothmure present he explained his difficult position. He is sworn to House Tully, who has declared for the rebels, but could not bear it if his forces caused harm to his brother Oswell of the King’s Guard. Ultimately he agrees to take up the rebel cause if Teddy can prove he is just and moral. He requests that the party “happen by” Saltpans because Sheera Cox has taken ill and they have no Maester. Ser Quincy is a proud man who would not accept help if it came from Walter (Walter and Quincy’s fathers were bitter rivals in their youths while squiring at River Run and that grudge lingers in Quincy)