In the A Song of Ice and Fire novels, Arys Oakheart is a member of Robert Baratheon's Kingsguard, appointed after his predecessor - whose name hasn't been mentioned - perished during the Greyjoy Rebellion. He's a member of House Oakheart, a noble house from the Reach.
He is regarded as one of the kinder Knights of the Kingsguard under King Joffrey, objecting at first to Joffrey's orders to beat Sansa Stark, then only hitting her as lightly as he dares. He is also of a much more pleasant demeanor, often engaging in pleasant conversation with the Stark girl.
Arys remains in Dorne as Myrcella's sworn shield. During this time, Arianne Martell, Prince Doran's daughter and heiress, seduces him. She persuades him to take part in her plan to crown Myrcella to incite war with the Iron Throne in retaliation for the death of Oberyn Martell.