Fisher King is the traditional title of the guardian of the Holy Grail. At present, it is held by Peredur fab Ragnal.  King Pelles was presumably a past holder of the title, since he reigned at Castle Carbonek. 
Real World Background
The Fisher King is traditionally the title of the Holy Grail's guardian in Arthurian legend, although opinion is divided as to why this is. According to the legends themselves, fishing was his chief pleasure, and the first Fisher King, Bron, was even noted for being able to catch miraculous fish that provided sustenance for the Company of the Grail. Arthurian scholars are less certain as to the reason, theories varying from the fact that the French words for "fisher" and "sinner" are extremely similar and the Fisher King is a sinner, thus suggesting that some word-play may be involved here, to the notion that the Fisher King is a literary descendant of Bran the Blessed, who was the son of Llyr, a Welsh sea-god.
The most prominent Fisher King in the Arthurian legends is Percival's uncle and predecessor, who was wounded in the leg with a magical spear for a sin that he committed (the exact nature of his offense varying from one version of the story to the next), a wound which refused to heal until either Percival or Galahad cured him; he is often identified with King Pelles.
- Fisher King at Wikipedia, the Free Encyclopedia