The knight who seeks the Grail and, generally, becomes its new keeper upon achieving it.
In the earliest stories of Chrétien, Wolfram, and their adapters, the Grail Hero is Perceval (or, in Heinrich von dem Türlin’s Diu Crône, Gawain). The Vulgate Queste del Saint Graal and later stories, however, give the primary honor to Galahad and put Perceval in a secondary position, along with Bors.
Sources
Perceval, or Le Conte del Graal | Chrétien de Troyes, late 12th century
Parzival | Wolfram von Eschenbach, 1200–1210
Diu Crône | Heinrich von dem Türlin, c. 1230
Vulgate Queste del Saint Graal | 1215-1230