Gerold Hightower

Gerold Hightower is an unseen character in Game of Thrones. He is not expected to appear in the series, being long dead by the time it starts. Ser Gerold Hightower was a knight of House Hightower and the Lord Commander of the Kingsguard under Aerys II Targaryen.

Season 4
Ser Gerold Hightower's entry in the Book of Brothers mentions several events of his life. Among them are his injury defending the honor and property of Princess Elia Martell; granting temporary command of the Kingsguard to his sworn brother Ser Arthur Dayne. The Book also mentions he was sent to located Prince Rhaegar Targaryen on the wake of Robert's Rebellion and his death along with Ser Arthur and Ser Oswell Whent at the Red Mountains of Dorne fighting a small force led by Eddard Stark after refusing to bend the knee to the new king, Robert Baratheon.

In the books
In the A Song of Ice and Fire novels Ser Gerold Hightower was known as "the White Bull" for his immense strenght. He was Lord Commander of the Kingsguard during the brief reign of King Jaehaerys II and then under King Aerys II.