Rhaegar Targaryen

"Your brother Rhaegar was the last dragon."

- Jorah Mormont to Daenerys Targaryen

Rhaegar Targaryen is an unseen character in Game of Thrones. He is deceased when the events of the series commence. He was the son and heir of King Aerys II Targaryen, holding the title of Prince of Dragonstone. Rhaegar was the older brother of Viserys and Daenerys, though Daenerys was born after he died. He is blamed for starting Robert's Rebellion by abducting Lyanna Stark. He was killed by Robert Baratheon during that conflict.

Background
Prince Rhaegar Targaryen was the son and heir to the Mad King, Aerys II Targaryen. He sparked the war known as Robert's Rebellion when he kidnapped Lyanna Stark of Winterfell for reasons unknown. He was killed by Lyanna's betrothed Robert Baratheon in single combat at the Battle of the Trident. When Rhaegar was killed, he was struck down with a massive blow from Robert's warhammer, which scattered the rubies encrusted in Rhaegar's armor under the water. That location was named the "Ruby Ford" thereafter.

Season 1
Lyanna's brother, Eddard Stark, and her betrothed, Robert Baratheon, joined forces with several other houses to destroy the Targaryen dynasty. Rhaegar himself met Robert in combat at a ford during the Battle of the Trident, and was slain by him there.

His father was killed shortly afterwards by Ser Jaime Lannister during the Sack of King's Landing. His younger brother Viserys and sister Daenerys (who was born after Rhaegar's death) survived in exile in the Free Cities across the sea. Lyanna also died during the war.

In the books
In the A Song of Ice and Fire novels, Rhaegar is depicted as a chivalrous, honorable, well loved by the smallfolk and just warrior, albeit one often distracted by other concerns and apparently unable or unwilling to restrain the worst excesses of his father. Rhaegar had silver-gold hair and dark lilac eyes. He was considered to be tall and handsome. Cersei Lannister, who was infatuated with him, remembers Rhaegar as the most beautiful man she had ever seen. Rhaegar was very bookish in his youth, such that people jested that the Queen swallowed some books and a candle while he was in her wombs. As a boy he was able to impress the maesters with his wit. He was married to Princess Elia Martell of Dorne, with whom he had two children, Rhaenys and Aegon. His reasons for kidnapping Lyanna Stark are unclear, but it is known that they had met at a great tourney at the castle of Harrenhal a year before the kidnapping. Rhaegar had crowned her Queen of Love and Beauty, passing over his wife.

One of the main reasons that the other noble Houses of the realm put up with King Aerys' increasing madness for as long as they did is because Rhaegar was widely loved and respected by all, and it was believed he would make a great king. Most were willing to simply wait out the rest of Aerys' reign until his son succeeded him, rather than deal with the controversy of rebelling against and killing a king. It was seen as a great shock when Rhaegar absconded with Lyanna Stark, but even so, royalists in the civil war weren't primarily fighting out of loyalty to Mad King Aerys, but out of loyalty to Prince Rhaegar. Thus when Rhaegar died in the Battle of the Trident, the Targaryen cause was doomed, and not simply due to a decisive military defeat. Without Rhaegar, Aerys' allies began to slip away, and it was only after the defeat at the Battle of the Trident that the Lannisters turned on the Targaryens, resulting in the Sack of King's Landing and the death of Aerys.

When Daenerys Targaryen was in the House of the Undying, one of the visions she saw was: a man resembling Viserys but taller than him, with dark indigo eyes, and a woman nursing a newborn babe in a great wooden bed. They are believed to be Rhaegar, Elia, and their son Aegon. In the vision, the man said that the infant "is the prince that was promised, and his is the song of ice and fire", and "The dragon has three heads".