Popa was the pagan wife or concubine he repudiated in order to marry the daughter of the King of France. When she died childless, Rollo resumed his relationship with Papie.

From "Debrett's Kings and Queens of Britain" by David Williamson, ISBN 0-86350-101-X, p. 42:
The ducal House of Normandy, which acquired the English throne by conquest in 1066, owed its origin to Rolf or Rollo the Ganger, a Norse sea-rover who carried out a number of raids on the coast of northern France. He was finally granted the Duchy of Normandy as a fief of the crown from Charles the Simple, King of France, accepting Christian baptism at the same time. Rollo's ancestors can be traced with near certainty to the early Yngling kings in Sweden. Rollo is credited with having contemptuously tipped the French monarch backwards off his throne when he came to do homage for his fief and he and his successors were well-nigh independent sovereigns, only paying lip-service to the French crown.

After a career of depredation, he had been recognized as a legitimate ruler in Neustria by the emperor, Charles III 'the Simple' about 911, and had thereafter passed on his power in an unbroken succession to his son. Rolf, known to his Frankish posterity as Rollo, was probably of Norwegian stock, being the son of Rögnvald, earl of Möre, and before his formal establishment in Gaul he had a long career as a Viking, raiding not only in France but also in Scotland and Ireland. In 911, having entered Gaul afresh, perhaps by way of the Loire valley, he was defeated in a pitched battle outside the walls of Chartres, and it was after this that he and his followers were given lands by the emperor in the valley of the lower Seine. Before 918 Rolf and his followers already held considerable lands in this region, and they had been formally confirmed in possession of them by the emperor.
In token of the new position he was henceforth to occupy in Gaul, Rolf accepted baptism at the hands of the archbishop of Rouen. His power was steadily to grow. The earliest demesne of the dynasty was confined to an area bounded by the Epte, the Orne, and the sea: it was concentrated in the district lying on both sides of the Seine between Les Andelys and Vernon, and stretched to the west nearly as far as Évreux, and to the east along the Epte towards Gisors. Between 911 and 918 Rollo was also in possession of Rouen and certain districts on the sea-coast dependent on that city, and by 925 he was established as far to the east as Eu.136

Despite the baptism of Rolf, the Viking dynasty itself only slowly renounced the traditions of its pagan past. It is not impossible that Rolf reverted to paganism before his death.136

The grant by Charles the Simple to Rolf vested the Viking leader with some at least of the rights and responsibilities of a Carolingian count, and it is certain that 'count' was a title much favoured by early members of his family.136

From Rosamond McKitterick's, "The Frankish Kingdom under the Carolingians, 751-987", London & NY (Longman) 1983, pp 237-238:
"It is not known when Rollo arrived in the Viking kingdom [in Normandy]. Dudo says that he took Rouen in 877, but most historians are agreed that Rollo probably did not appear in Francia until the early tenth century. The possibility exists however, that Dudo is preserving a belief that Vikings had been established in the Rouen area from about this time. Rollo is thought to have been Norwegian rather than Danish, and later Icelandic sources identify him with Hrolf the Ganger (walker), son of RAGNVALD EARL OF MOER, who had a career as a Viking before settling in Francia. He married a Christian woman and his son William, according to the Lament of WILLIAM LONGSWORD, was born overseas. (P) Nothing more is known about the 'Treaty of St Clair-sur-Epte' concluded in a personal interview between CHARLES THE SIMPLE and Rollo than Dudo tells us, and he has been accused of inventing the meeting. That a cession of territory in the Seine, extending as far west as the mouth of the Seine on the coast and near the source of the Eure inland is affirmed by a charter of CHARLES THE SIMPLE dated 14 March 918. ..... Flodoard adds the information that Rollo received baptism and the Frankish name Robert with the cession of this territory. (P) Rollo seems to have been made a count in 911, with the traditional duties assigned to a Carolingian count, namely, protection and the administration of justice. He was certainly subordinate to THE FRANKISH KING. With the proliferation of titles accorded the leader of the Normandy Vikings in later sources, some
historians have suggested that Rollo was made a duke, but Werner has argued that there was no Norman 'marchio' before 950-6, and no duke before 987-1006, that is, after HUGH CAPET had gained the throne of France. ..... (P) Rollo appears to have received his territory on similar terms as the Bretons had received the Cotentin, except that the bishoprics were also ceded. ..... In exchange, Rollo was to defend the Seine from other Vikings, accept baptism and become the 'fidelis' of THE FRANKISH KING. That there were other groups of Vikings in the region, particularly in the western part of Normandy, is clear. The west stayed pagan longer; it was a century before a bishop was appointed to the Cotentin. ..... (P) The arrangement made in 911 proved successful ..... The area of Normandy by 933 corresponded to the area of the archdiocese of Rouen, with the seven *civitates* of Rouen, Bayeux, Avranches, Evreux, See's, Lisieux and Coutances. The fortunes of the bishops of Rouen and of the 'principes' of Normandy were in fact closely associated from the very beginning."