Guillaume was the son of Bonabes I de Derval and Eustaice. The date and place of his birth have not been found.
His wife is not known. They were married, but the date and place have not been found. Their only known child was Guillaume II (?-?).
Guillaume I de Derval
Bonabes I de Derval
Noël de Derval
Marie de Rostrenen
Eustaice