Although his filiation has not been confirmed, several hypotheses have been put forward. Medievalist scholar Margarita Torres Sevilla-Quiñones de León believes that he could have been the son of Nuño Ordóñez, a son of King Ordoño I of Asturias and brother of Alfonso III, which, according to the author, would explain the rapid rise of this lineage and its proximity to the royal house.
According to another hypothesis, proposed by genealogist Jaime de Salazar y Acha, Bermudo would have been the son of a Nuño Vélaz (or Vela), a son of Vela Jiménez, Count of Álava, through whom the Vela lineage spread throughout the Kingdom of León which would also explain the Basque-Navarrese names in subsequent generations.
Bishop Oveco Núñez in a charter dated 28 August 945, confirms several donations made by King Ramiro II of León to the Monastery of Sahagún, also confirming the charter his brothers Vela, Suero, Munio, Bermudo and Nuño Núñez, followed by some of the children of these brothers, including Fernando Bermúdez, son of Bermudo Núñez. He also had a sister named Gontrodo who married Ansur Fernández, Count of Monzón, the parents of Queen Teresa Ansúrez, wife of King Sancho I of León.