Rollo (c. 846 – c. 931), baptised Robert and so sometimes numbered Robert I to distinguish him from his descendants, was a Norse nobleman of Norwegian or Danish descent and founder and first ruler of the Viking principality which soon became known as Normandy. His descendants were the Dukes of Normandy, and by later extension, the King of England.
The name "Rollo" is a Latin translation from the Old Norse name Hrólfr, modern Scandinavian name Rolf (cf. the latinization of Hrólfr into the similar Roluo in the Gesta Danorum), but Norman people called him by his popular name Rou(f) (see Wace's Roman de Rou). Sometimes his name is turned into the Frankish name Rodolf(us) or Radulf(us) or the French Raoul, that are derived from it.
Read more about Rollo: Historical Evidence, Raids Along The Seine, The Principality of Normandy, Family, Death, Legacy, Depictions in Fiction