Geoffrey of Monmouth presents Cador as Duke of Cornwall and a leading supporter of Arthur. His position belongs to the imagined political world of the History of the Kings of Britain, completed in the twelfth century. Later writers and Welsh traditions vary his ancestry and precise relationship to Arthur, so descriptions of him as a cousin, nephew or half-brother should not be forced into one consistent family tree. The Welsh form Cadwr, written Kadwr in Charlotte Guest's translation, connects him with the figure who arms Arthur in The Dream of Rhonabwy. Similar names elsewhere do not by themselves prove that all the bearers are the same man.

Cador first proves his military importance in Geoffrey's account of the Saxon wars. Arthur sends him against an enemy force attempting to relieve the army near York. After Arthur's later victory at Bath, Cador receives the task of pursuing the fleeing Saxons while the king turns to aid Hoel. Cador first captures their ships, cutting off a route of escape, and then follows the survivors to Thanet. Geoffrey's narrative associates the final defeat with the death of their leader Cheldric, also written Chelric. This sequence distinguishes Cador's pursuit from Arthur's earlier negotiation of a settlement that the Saxons subsequently broke.

The duke's household also forms part of the royal marriage story. Geoffrey says that Guinevere was brought up there before she married Arthur. The description of noble Roman ancestry in that passage belongs to Guinevere, not to Cador himself. His guardianship links him to the queen's early life without making him responsible for every later version of her story. In the continental-war tradition he continues to act as an experienced commander, and Malory includes him in the escort of prisoners threatened by an enemy ambush. His narrative role is therefore broader than that of a local lord who appears only in Cornwall.

Welsh literature gives Cador a more intimate form of authority. In The Dream of Rhonabwy, confusion spreads through Arthur's army as the warriors watch Kay ride. A call goes out for Kadwr, Earl of Cornwall, who rises holding the king's sword. Its golden serpent ornamentation seems to release flames when the blade is drawn, and the tumult falls silent. Rhonabwy's guide explains that this is the man whose duty is to arm Arthur for battle. The scene does not need to describe a separate military victory: Cador's place beside the king is made visible through a ceremonial action that commands the attention of the host.

Cador's final importance lies in the succession after Arthur's last battle. Geoffrey names Constantine, the son of Cador, as the ruler to whom the wounded Arthur entrusts the crown. Traditions of the final fighting include Cador among the fallen, but obscure names in lists of casualties should not be converted into secure personal titles without explanation. Constantine's subsequent reign also differs between chroniclers. These variations leave the central relationship intact: Cador connects the king's military household, Guinevere's upbringing and the next ruler. His story is best read as an Arthurian literary biography rather than as a reconstruction of an independently established sixth-century duke.