Lewyn Martell
A prince of Dorne in a white cloak, and Princess Elia's uncle: sworn to guard a king who was keeping his own niece and her children in the capital as hostages in all but name.
A soul of House Martell, born at Sunspear.
The fuller tale
Their story
Prince Lewyn was a Martell of Sunspear and uncle to Princess Elia, and he took the white cloak — which meant giving up his lands, his name's inheritance and any prospect of a wife or children, all of which a Dornish prince has rather more of than most men who swear that vow.
The position he ended up in is the worst any Kingsguard has ever occupied, and it was not his doing. His niece and her two small children were kept in the Red Keep through the whole rebellion, and everyone in Westeros understood exactly what they were there for: Dorne's fifty thousand spears would come when called, because the alternative was what a mad king does to hostages. Lewyn was in that castle. He knew what his niece was. He wore the cloak anyway.
It is worth being clear that he was well liked, and not merely tolerated. The Dornish followed him gladly. He was said to keep a paramour, which the vow forbids and which nobody in Dorne considered a scandal and nobody outside it seems to have raised with him twice.
How it ends for them
Fell on the Trident, before the story — before this story begins.
He commanded the Dornish spears at the Trident, took a wound that should have finished him, refused to leave the field, and was killed there — a man keeping a vow to an institution that had stopped deserving it years earlier, in a battle fought to defend a prince who had ruined his niece's life.
Elia and her children outlived him by weeks. What was done to them in the sack is the reason Dorne's grievance is not a diplomatic position but a wound, and it is still open a generation later when this story starts.