Yohn Royce is the Lord of Runestone and a powerful bannerman of House Arryn and House Stark.
Yohn Royce is present in the tribunal, hearing the testimony of Lord Petyr Baelish about the death of Lysa Arryn. Along with Lady Anya Waynwood and Ser Vance Corbray, he assumes foul play on Littlefinger's part. He openly expresses his dislike of Littlefinger, calling him a whoremonger, money-grubber, and liar, as well as pointing out his tendency to suck up to Tywin Lannister in the past. The tribunal then summons Littlefinger's niece, "Alayne", the only other witness. Lord Yohn is surprised to learn of Alayne's true identity - Sansa Stark. Satisfied with her testimony, Royce and Waynwood discuss the Vale's next move with Baelish. He suggests that it is long past time that Robin left the Eyrie and conducted an official tour of the Vale.
Royce agreed with Petyr Baelish to foster Robin Arryn at Runestone, although he is not impressed with the skills of the young lord.
Royce oversees Robin Arryn's archery training, watching in contempt as Robin fails to hit the marks. The training is interrupted when Baelish arrives by carriage. Royce is outraged as Baelish told him that he and Sansa Stark would return to the Fingers, yet Sansa was married to Ramsay Bolton at Winterfell. Baelish uses the opportunity, falsely claiming that on the way to the Fingers they were attacked by a vast Bolton host which seemed to have inside knowledge of Baelish's travel plans. Baelish reveals that Royce was the only one he told about his route and begins casting aspersions on his loyalty, asking Robin how to fare with him. Although Robin at first plans to throw Royce out of the Moon Door, Baelish advises him to spare Royce because of his loyalty to House Arryn. Instead, Royce is named commander of the armies of the Vale and told to round up the Vale's soldiers in order to march North and save Sansa from Ramsay.
The Vale reinforcements win a crushing victory against the Bolton infantry, slaughtering the phalanx surrounding the depleted Stark forces and allowing Jon Snow to liberate Winterfell with a wildling force.  Lord Royce later voices disgust at the Knights of the Vale allying themselves with "Wildling invaders". He watches as Lady Lyanna Mormont, Lord Wyman Manderly, and Lord Robett Glover proclaim their allegiance to Jon Snow as the new King in the North, and declares allegiance to House Stark along with the other Lords of the Vale.
A lord of the Vale and a descendent of one of Westeros' oldest families, Lord Royce met Sansa Stark at Winterfell when he escorted his son Waymar to join the Night's Watch. Initially distrustful of Littlefinger's motives, he was swayed by sansa's testimony on Lord Baelish's behalf. Lord Royce is fostering Robin Arryn at his family's castle.