Jaehaerys I Targaryen

JaehaerysI Targaryen is an unseen character in Game of Thrones. He is not expected to appear in the series, being long dead by the time it begins.

Background
Jaehaerys I was the fourth king of the Targaryen dynasty, and reigned two centuries before the War of the Five Kings.

Season 3
Trying to make his way back south of the Wall, Samwell Tarly explains to Gilly that there is a secret passage at the Nightfort, the first castle on the Wall, and that the castle was abandoned during the reign of King Jaehaerys the First.

In the books
In the A Song of Ice and Fire novels, Jaehaerys I is fondly remembered as one of the best kings the Targaryen dynasty ever produced, and his long reign became synonymous with peace and prosperity. He was the longest-ruling king in history, sitting on the Iron Throne for fifty-five years, and due to his advanced age by the end of his reign is often called "the Old King". He is also known as "Jaehaerys the Conciliator" because he brought an end to the eleven year uprising by the Faith of the Seven which had been fought by his uncle Maegor the Cruel. Jaehaerys offered the Faith generous terms of amnesty, so long as they acknowledged Targaryen rule and disbanded the military orders of the religion, known as the Faith Militant.

Jaehaerys I was the son of Aegon I's older son, Aenys I, a weakling who did not easily hold onto the throne. During Aenys' reign his younger half-brother Maegor served as Hand of the King and was the real power behind the throne. Through unspecified events Maegor somehow succeeded his brother ahead of his nephew Jaehaerys, possibly through openly seizing the throne in a coup after secretly holding the real power for so many years. Maegor eventually died childless, however, which allowed Jaehaerys I to succeed him.

Jaehaerys I ruled wisely and well until the end of his long reign. Indeed, Jaehaerys lived so long that he actually outlived his own son (who may have died young), and was thus succeeded by his grandson Viserys I (the only time in history that a king was succeeded by his grandson).