Final answer:
Sir Modred ruled Britain while Arthur was in France, usually depicted as a traitor in Arthurian legend.
Step-by-step explanation:
The character who ruled Britain while Arthur was in France, according to Arthurian legend, is Sir Modred.
This question pertains to tales of King Arthur, which are part of English literature and folklore. Sir Modred, who is often portrayed as a traitor and a usurper, took control of the throne in King Arthur's absence. This action typically leads to conflict upon Arthur's return, resulting in a final battle between Arthur and Modred.