- (c. 1338-1375) Galician nobleman, Fernán Ruiz de Castro, Count of Trastámara, and Lord of Lemos y Sarria. Son of Pedro Fernández de Castro and his second wife, Isabel Ponce de León.
- Served Pedro of Castile as chief lieutenant (adeantado-mor) and chief provincial governor of León and Asturias. [Pedro, ch. 40: 11-12]
- Received the titles of Count of Trastámara and Lord of Lemos y Sarria from Pedro of Castile in 1366. These were the titles of Enrique, the half-brother of the monarch, but since he had invaded Castile, Pedro invested them on Fernán de Castro, his faithful vassal during the civil war, and made them hereditary. [Pedro, ch. 40: 35-39]
- The Castro were a very influential Galician family. Fernán de Castro is also the brother of Juana de Castro, married to Pedro I of Castile (1354).
- He is also half-brother to Inês de Castro, made immortal in many literary works on account of her love affair with Pedro, future king of Portugal, and her untimely death by direct or indirect command of King Afonso IV of Portugal.
- Enrique, Count of Trastámara, encourages him to join in on an invasion of Castile together with Fadrique, the Master of Santiago, and João Afonso de Albuquerque. [Pedro, ch. 17: 122-124]
- Remained loyal to Pedro of Castile, holding the castle of Lugo in his name. Enrique II besieged the city ut was unsuccessful. [Pedro, ch. 41: 39-41]
Fernán Ruiz de Castro