Who is Ser Arys from A Clash of Kings and what is their importance?

Ser Arys is a knight of the Kingsguard. He is kinder to Sansa Stark than the other members of the Kingsguard. Joffrey frequently has his Kingsguard beat Sansa, and when he ordered Arys to do it, Arys did his best to make it easy on her. He takes Sansa down to the tourney celebrating Joffrey's birthday. He is sent with Myrcella Baratheon when she leaves for Dorne to protect her.