Gytha

Countess Gytha was the mother of King Harold and the sister of King Cnut of Denmark. She and her daughter-in-law Edith Swan-Neck falsely identified the body of her son on the battlefields of Hastings on 14 October 1066. This lie allowed her son to be "reborn" as Hereward the Wake, a formidable Saxon guerrilla warrior. (PROSE: The Real Hereward)

Behind the scenes
Historically, Cnut was almost certainly not Gytha's brother, and likely not blood-related to her at all. However, there was a definite attempt to invent a blood relationship between the two. The information in The Real Hereward comes from one of Hereward's men, which is entirely logical. The whole point of the subterfuge was to make the average Saxon believe in some sort of connection between Gytha and Bluetooth, Cnut's grandfather, the founder of the then-modern Danish/English monarchy. That one of Hereward/Harold's men believed it meant that the propaganda effort succeeded.