Ser Barristan the Bold, Now called Barristan the Old, or Ser Grandfather, Lord Commander of the Queensguard
Ser Barristan is at once a true knight, and the exemplar of the failure of a knight. He was a zeal guardian to three different kings, risking life and limb to protect them from harm. He is one of the most skilled swordsmen alive, and is skilled in horsemanship and jousting. Not only is he skilled in combat, but he can train and lead other men, and advise rulers. Yet, when Robert Baratheon took the throne, Barristan's loyalty waivered. He took a pardon, and went on to serve Robert until his death, betraying the lineage he swore to protect. He only chooses to return to Targaryen service once he has been dismissed from the Baratheon Court.
Perhaps his foundation being shaken woke Barristan up, and reminded him of where his loyalty truly lied. But if his own dishonor is what it took, and not the murder of a woman and her two children, or the burning of king's landing, then I don't think I trust his values. It feels more likely that he saw the horrors of Aerys II, and decided that Robert was the better option. Perhaps he hoped that Viserys' time away from the throne had taught him a kindness that his father lacked. Barristan would have been sorely dissapointed.
Whatever his motives, Barristan has been a loyal servant to Daenerys. Even now that she is missing, he refuses to believe her dead, and acts to maintain her version for Meereen. Whether or not Daenerys will approve of his actions is yet to be seen.
