Maekar Targaryen

"My father was Maekar, the First of his Name. My brother Aegon reigned after him, when I had refused the throne, and he was followed by his son Aerys, whom they called the Mad King."

- Maester Aemon

Maekar I Targaryen, full name Maekar of House Targaryen, the First of His Name, is a mentioned character in the first season. He died before the time of the series, and is not expected to appear. He was a King in the Targaryen dynasty.

Maekar's personal sigil was the normal Targaryen sigil, quartered, so that it repeated four times.

Background
Maekar was a King in the Targaryen dynasty. Unlike the majority of their forbears, Maekar and his siblings - Baelor, Aerys, Aelinor, and Rhaegel - were not born out of marriage between brother and sister. Instead, their father Daeron II and his sister Daenerys respectively wed Myriah and Maron Martell in order to unite Dorne with the Iron Throne. Since his only sister Aelinor married Aerys, Maekar wed another woman of Dornish nobility, Dyanna of House Dayne. He was the father of Aerion, who ultimately killed himself drinking wildfire; Aemon, who would become a Maester at Castle Black and a member of the Night's Watch; and Aegon V Targaryen, who would succeed him to the Iron Throne. Although he was the fourth son, it eventually fell on Maekar to assume the throne and further the Targaryen dynasty after the subsequent deaths (one of which was actually caused by Maekar himself) of his three elder brothers. Maeker killed his oldest brother Baelor in the Trial by seven of Duncan the Tall, which was ironically provoked by his eldest son Aerion.

Season 1
When revealing his true identity and origins to Jon Snow, who unbeknownst to them both is also directly descended from Maekar, Maester Aemon mentions the name of his father.

Season 6
Eventually, Bran Stark discovers that his bastard "half-brother" Jon Snow is actually the son of his aunt Lyanna Stark and Rhaegar Targaryen, meaning Maekar was in fact Jon's true patrilineal great-great-grandfather.

In the books
In the A Song of Ice and Fire novels, Maekar Targaryen is the fourteenth Targaryen to sit the Iron Throne. He was the fourth son of King Daeron II and Myriah Martell. As the son of Daeron II, he was also the grandson of King Aegon IV Targaryen. Maekar was given the Targaryen seat of Summerhall, and was a capable army commander who served in the Blackfyre Rebellion. After a number of unexpected deaths in the royal family, he came to the throne in 221AL. He ruled until 233AL, when he fell in battle against a rebel lord.

He was the father of Daeron, Aerion, Aemon, Aegon, Rhae and Daella.

Maekar appears as a major character in the Tales of Dunk and Egg prequel novellas, and would appear in any potential live-action adaptation.