King Rhaegar I Targaryen is the rightful king of Westeros. He is the son of King Aerys II Targaryen (Also Known as the Mad King) and Queen Rhaella Targaryen. Rhaegar was married to Elia Martell whom he had a contentious relationship with. In 281 AC, he met and fell in love with Lyanna Stark. The two were married in 282 AC after he annulled his marriage to Elia.
The action led to a war where Lyanna's former betrothed, Robert Baratheon, toppled his dynasty. Rhaegar was unaware of the rebellion until his father sent Ser Gerold Hightower to find him and Lyanna, on their honeymoon, to make Rhaegar return to King's Landing to fight in the war. Rhaegar led the loyalist forces to battle Robert, and they fought on the River Trident. Rhaegar was defeated, but he was saved by a loyalist faction, who took him to a maester to recover.
After the rebellion, Rhaegar fled to Essos and took the alias Vaegon Maeterys to help him look for his siblings. He had heard no word of Lyanna or their child, so he was led to believe she was dead. He also began plotting to regain his family's throne.