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.
Royce is present at King Snow's meeting regarding plans against the White Walkers' impending invasion. He opines that Last Hearth and Karhold ought to be totally demolished in retribution for House Karstark and House Umber's treason against House Stark. However, Sansa and Jon disagree with this and Jon ultimately allows Ned Umber and Alys Karstark to retain their ancestral homes after renewing their pledge of loyalty.
Royce speaks out against Jon's plan to treat with Daenerys Targaryen, on the basis that "a Targaryen cannot be trusted" unaware of Jon's own true parentage.
Yohn is ordered by Sansa to see that the armor made for their armies are outfitted with leather to keep warm.
Later, Lord Royce joins Lord Robett Glover in calling for Regent Sansa to take over from her half-brother Jon Snow as ruler of the North. Sansa rejects their petition and insists that Jon is their true ruler and that she is only a regent. Arya Stark suggests that Sansa assassinate the troublesome lords but Sansa rejects the idea as repugnant.
Lord Royce is later present at what is ostensibly a drumhead trial for Arya Stark. Royce is clearly not surprised or displeased that the trial turns out to be Littlefinger's instead. Sansa confronts Littlefinger with his countless crimes against the Starks, Arryns, Tullys, and many others who have suffered from the massive war he discreetly started. When Petyr demands that Lord Yohn escort him back to the Eyrie, Royce scornfully refuses, signaling that he no longer recognizes his authority. He later watches impassionately as Lord Baelish is executed by Arya Stark, ending Littlefinger's machinations once and for all.
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.