Jacaerys Targaryen of Whitefyre "The Cruel"

Jaecaerys was the son of Maelor I, the Usurper and just like his father he considered himself the rightful heir. At the death of his father, Vaegon I, his cousin and rightful heir to the Throne of the Three Islands before Maelor took it from the infant, was crowned. Jaecaerys then rose in arms to usurp the position from Vaegon as son of Maelor. In reality the coronation was done as soon as Maelor I died, quickly to avoid having to face Jaecaerys.

Jaecaerys was very similar to Maegor Targaryen, the third Targaryen King.