Petyr is a knight and member of House Arryn of Gulltown, a wealthy but extremely distant branch of House Arryn in the Vale's largest city. He has a great disdain for anything remotely close to responsibility.
Serving as a Knight of the Vale, Petyr eventually made the acquaintance of Nathaniel Arryn and began serving the Warden of the East as a household knight to the Lord when he traveled to King's Landing to serve as Hand of the King. The two men developed a close friendship while in King's Landing and later the Eyrie, despite being near polar opposites.
In 510 AC, Petyr refused Nathaniel when offered a position in the newly formed Brotherhood of Winged Knights as its first Commander. Instead, he insisted upon remaining a simple household knight.
During Sunderland's Rebellion in 513 AC, Petyr was amongst the leadership in Nathaiel's host sent to put down the rebells. He commanded the Arryn's portion of the Vale fleet, alongside Lord Garrett Grafton whom he had a strong childhood dislike for. Following the Sacking of Sisterton and the injuring of Nathaniel, Petyr was named a member of the Lords Declarant for the young Theon Arryn to assist him in ending the war. He did so, sitting in on Theon's first court and helping deal out justice to the Sistermen. When instructed to take up the new role of Castellan of Sunderland Keep, however, Petyr was furious and walked out of court. He took the position, but was enraged with Theon and the other Lords Declarant for the decision.