Arys Oakheart was a sworn warrior of the Kingsguard to King Robert Baratheon and later Joffrey and Tommen. At the order of the then-acting Hand of the King, Tyrion Lannister, he escorted Princess Myrcella Baratheon to Dorne, where she had been sworn in marriage to Prince Trystane Martell.
Arys was seduced by Princess Arianne Martell, who planned to crown Myrcella as Queen of the Seven Kingdoms to trigger a war between the Martells and Lannisters to avenge the death of her aunt, Elia, and uncle, Oberyn, at the hands of Lannister bannermen. Arys was persuaded to join this conspiracy. However, the plan was exposed and Arys was killed in battle by Areo Hotah, Prince Doran's captain of the guards.
Arys was a minor POV character in A Feast for Crows where he had a single chapter, entitled 'The Soiled Knight'. He is, so far, the only non-prologue POV character to have only a single chapter before perishing.