Arryk Cargyll

"Twin brothers even ended up on opposite sides; died with tears on their cheeks as they cut each other down."

- Bronn

Ser Arryk Cargyll is an unseen character in Game of Thrones. He is long dead before the start of the series, and thus is not expected to appear. Arryk Cargyll was a renowned knight, and member of the Kingsguard who fought and died in the Dance of the Dragons.

Background
Ser Arryk was the twin brother of Ser Erryk Cargyll, who was also a member of the Kingsguard.

The Kingsguard (Histories & Lore)
Ser Arryk and Ser Erryk Cargyll are mentioned as being twin brothers who ended up on opposite sides of the Targaryen civil war known as the Dance of the Dragons. The twin brothers supposedly met on the field of battle, and died at each others hand with tears on their cheeks.

In the books
In the A Song of Ice and Fire novels, Ser Arryk Cargyll was also the twin brother of Ser Erryk, and both were knights of the Kingsguard.