Prince Illyrio or "Good Prince" Illyrio was the Prince of Pentos during most of the War of the Brothers.
Illyrio was responsible for checking a period of decline in Pentos. Some say he helped put the city back on the path to greatness. When the Thousand Banners headed for Myr, he knew that Pentos would be next. The Magisters would not let anyone save the Prince lead their armies, but he was not a military man, so according to legend, he let himself be killed so that his successor could help stop the Brothers Malevolent.
There is a famous song written about his death called Illyrio's Lament.