Alternate: Judicael Berengar

Stuart's "Royalty For Commoners" (334:36) referes to him as Count of
Rennes and a leader against the Norsemen.

Per "The Bretons" by Galliou and Jones, a 15 year remission from Viking raiders in Brittany brutally ended in 913. In the next few years several Viking states, similiar to the one forming in Normandy, were established in Brittany. The county of Nantes was abandoned to the Vikings in 921 by KING ROBERT I of France. WILLIAM LONGSWORD, Duke of the Normans, aqdvanced through the Contenin and the Avaranchin. About 936, WILLIAM assented to the return to Brittany of Alain
Barbetote, Count of Cournaille from England - the same year that LOUIS IV D"OUTREMER returned to France from England to reestablish Carolingian rule there. An earlier attempt by Alain to regain
his inheritance in 931 had failed. While other Breton counts were seeking refuge at the English court, Berenger of Rennes alone remained to withstand the Viking onslaught. He was surrounded by them. Direct contact between the rulers of Brittany and the Kings of France gradually ceased in the course of the 10th century. No Duke of Brittany acknowledged fealty of performed homage directly to the King of France until 1099. Following Alain Barbatorte's death in 952, leaving no clear heir, a power vacuum in the duchy was filled by the Counts of Rennes.

"Todd A. Farmerie" posted to soc.genealogy.medieval on 22 Nov 1996 (in part):
Subject: Re: ancestry of CONAN I, Duke of Brittany
"I still follow the work of Lot and others from the turn of the century,
who show Juhel/Judiceal Berenger as son of a Count Berenger, a Frank
who rooted in Brittany toward the end of the 9th century, and who is
perhaps identical with the Berenger of Bayeux that Rollo ran into. I am
deeply suspicious of any attempt to harmonize all of the breton lines
into a single royal family, suspecting instead that from the time of
Nominoe, there were several entities (i.e. Vannes, Nantes, etc.) of which
the king/count was simply the most powerful at the time."

BERENGER, COUNT OF BAYEUX (RIN 1314), father of ROLLO's wife,
POPPA, could, if Farmerie is correct, be his father.

References: [Theroff],[PlantagenetA],[ES],[Weis1],[AR7]