Jaehaerys I Targaryen was the fourth Targaryen King. He was the third son of Aenys I and Alyssa Velaryon, and a grandson of Aegon I Targaryen.
Jaehaerys was born in 34 AC. In 42 AC his father died and Aenys' half-brother Maegor "the Cruel" Targaryen usurped the Iron Throne due to the scheming of his mother Visenya Targaryen. Jaehaerys' eldest brother Aegon tried to claim the throne, but was slain in battle with Maegor in 43 AC. Jaehaerys, his sister Alysanne, and mother were kept hostage on Dragonstone by Visenya. However on her death they fled with the Valyrian steel sword Dark Sister. They found sanctuary at Storm's End. Maegor tortured his squire Viserys, Jaehaerys' older brother, to death.
Maegor married his niece Rhaena, widow of her brother Aegon, who gave birth to daughters Rhalla and Aerea. Maegor named Rhalla his heir to disinherit Jaehaerys.
However Maegor's tyrannical rule led to the Seven Kingdoms rising against him in 48 AC. Jaehaerys put forward a claim and was supported by the Lord of Storm's End, Robar Baratheon, who he named Protector of the Realm and Hand of the King. Maegor died mysteriously on the Iron Throne, unable to sire any children, meaning his nephew succeeded him. Jaehaerys was crowned at the age of 14 and married his sister Alysanne. Robar later married Alyssa and had a son, Boremund, and daughter, Jocelyn.
Jaehaerys ruled from 48 AC to 103 AC, longer then any other King on the Iron Throne. He inherited his uncles' war with the Faith Militant, partially over the Targaryens' incestuous practices. Jaehaerys sent his Hand Septon Barth to Oldtown to speak with the High Septon. In return for an offer of amnesty for all participants and the disbanding of the Faith Militant, and for agreeing to accept outside justice, the High Septon received King Jaehaerys's sworn oath that the Iron Throne would always defend the Faith of the Seven. For this Jaehaerys was known as the Conciliator. Jaehaerys took away the Faith's right to conduct trials, pleasing many due to the acts of unscrupulous Septons.
Jaehaerys greatest friend was Septon Barth, a blacksmith's son who worked in the Red Keep's Library and met Jaehaerys, who loved to read. Barth was the only common-born King's Hand (till Davos Seaworth two centuries later), serving for 40 years. With his aid Jaehaerys reformed the realm, creating a unified law code. King's Landing was improved with sewers, drains and wells as Barth believed it would improve the city's health. A network of roads were built to bind the realm together. Alysanne convinced Jaehaerys to abolish the lord's right to first night.
Jaehaerys and Alysanne had thirteen children, though only nine lived to adulthood. The eldest son, Aemon, married his half-aunt Jocelyn Baratheon, and had a daughter, Rhaenys, who married Lord Corlys Velaryon. However when Aemon died in 92 AC fighting pirates on the Island of Tarth, Jaehaerys named his second son Baelon was heir rather then Rhaenys (possibly as he had become King due to his elder brother's daughters being passed over), causing a quarrel with his wife, which was resolved in 94 AC with the help of their daughter Septa Maegelle. Alysanne died a few years before Jaehaerys.
In 101 AC Baelon died suddenly. and Jaehaerys called the first Great Council to determine succession. Baelon's eldest son by his sister-wife Alyssa, Viserys, and Rhaenys' son with Corlys, Laenor Velaryon, were the main claimants. Finally Jaehaerys' grandson was chosen over his female-line great-grandson, possibly partially as Viserys was 24 while Laenor was 7. Viserys was married to another of Jaehaerys' grandchildren, his cousin Aemma Arryn, from Jaehaerys' daughter Daella marriage to Lord Rodrik Arryn of the Eyrie.
Jaehaerys appointed Ser Otto Hightower, brother to Lord Hightower, as his Hand. In his last two years, Jaehaerys' mind began to wonder, and Otto's daughter Alicent looked after him. At the age of 69, he died while she was reading from Septon Barth's Unnatural History. His ashes were interred with Alysanne's and the whole realm mourned. Viserys succeeded his grandfather as Viserys I.