Ragnar is a Danish Earl and warlord who adopts Uhtred of Bebbanburg. Ragnar is the son of Ravn, a former earl and chieftain. He is the husband of Sigrid and the father of Ragnar the Younger and Thyra. Ragnar leads three ship's crews of Danes and is a lieutenant of Ubba Lothbroksson in the conquests of the Anglo-Saxon kingdoms of Northumbria, East Anglia, Mercia and the repeated attempts of conquering Wessex. Ragnar meets young Uhtred during the Battle of Eoforwic (modern York). Impressed and amused by the boy's bravery, Ragnar takes Uhtred captive and later adopts him as a step-son.
"Earl" is derived from the Norse princely title of 'Jarl' (pronounced as 'y-arl'). This aristocratic title would come to replace 'ealdorman' in Anglo-Saxon England.
Ragnar has a younger son called Rorik in The Last Kingdom book by Bernard Cornwell. Rorik was a childhood firend of Uhtred, though he was constantly ill from stomach cramps and died at the age of fourteen, leaving Ragnar grief-stricken.
Ragnar settles with his followers at Loidis (modern Leeds in West Yorkshire) in the BBC series, whereas they settled at Onhripum/Inhrypum (modern Ripon in North Yorkshire) in Bernard Cornwell's Saxon Stories.
Ragnar's followers would have been part of a larger Aett (Old Norse: 'kindred' or 'clan'). The military component would have been the Leidangr, which was the fleet-based expeditionary force mustered from the community. These were the type of forces which would have been called up during annual Viking expeditions. The smaller band of professional warriors which fought in an earl's service would have been called a Hird, who would have acted as household bodyguards most of the time. Earl Ragnar Ravnsson would have led his Leidang of three ships to Northumbria as part of the larger Danish force. Then called on the rest of his Aett in Denmark to come to England in the weeks after the Danish victory at Eoforwic.