Faustas
Son of Vortigern and brother of Vortimer, Catigern, and Pascentius. Faustus was Vortigern’s son by Vortigern’s own daughter, though P.F.J. Turner believes she was his step-daughter.
Nennius mentions him in connection with Saint Germanus, who baptized and raised him, and he eventually founded a monastery on the banks of the river Riez.
Source
Historia Brittonum | Probably Nennius, early 9th century