Aegon V Targaryen

A fourth son with no expectation of ruling, Aegon the Unlikely came to the throne through a chain of deaths nobody could have predicted and spent his reign trying to improve the lot of the smallfolk against the settled opposition of his own lords. He married Betha Blackwood for love rather than advantage, which tells you most of what you need to know about him.

He is the hinge of the whole lineage. His daughter Rhaelle married into House Baratheon and carried the dragon's blood to Storm's End; his direct line produced Aerys and then Rhaegar. Robert and the prince he killed at the Trident were both his descendants, which makes the rebellion a quarrel between cousins with a shared great-grandfather.