The Abbot of San Pedro de la Cardena. The original Poem of the Cid gives the name Don Sancho at this line, but there is no historical record of this person, although also nothing that denies his existence.
The recorded abbot from 1060 to 1086 was named Sisebuto, with his assistant from 1082 to 1086 being named Sebastian. There is no record of the names of their successors at the time of the events recounted in the poem.
The Abbot Don Sisebuto was saying matins, and Dona Ximena and five of her ladies of good lineage were with him, praying to God and St. Peter to help my Cid.
And when he called at the gate and they knew his voice, God, what a joyful man was the Abbot Don Sisebuto! Out into the court yard they went with torches and with tapers, and the Abbot gave thanks to God that he now beheld the face of my Cid.