FER-dee-ah
Cú Chulainn's foster-brother and closest friend, forced into single combat against him during the Táin and killed by the Gáe Bulg.
Ferdiad trained alongside Cú Chulainn under the warrior-woman Scáthach in Alba, and the two are described as inseparable in that time — foster-brothers bound by a closeness the later tale takes pains to establish before breaking it. When Medb needs a champion capable of matching Cú Chulainn during the Táin, she pressures Ferdiad into the role with promises of land, status, and his own daughter's hand.
Their combat, fought across several days at a ford, is the emotional center of the Táin: each night the two men send each other food, medicine for their wounds, and healing herbs, still friends even as they fight to kill each other by day. Cú Chulainn finally kills him with the Gáe Bulg, the only weapon capable of piercing Ferdiad's otherwise impenetrable horn skin, and his grief afterward is one of the most quoted passages in the whole cycle.