Harald Harefoot -- the name was a kenning referring to his speed and agility in the hunt -- was a son of King Canute of Denmark and England, and Aelgifu of Northampton. Unfortunately she appears only to have been a bed-warmer not a wife (Canute's wife being Emma of Normandy, widow to Aethelred Ill-Counsel or the Unready); Harold was accordingly illegitimate and Canute made his legitimate son, Harthacanute, his heir.
However, when Canute died, in 1035, Harold was able to seize power in England, whilst his brother was tidying up Denmark, and he obtained the position of Harthacnute's regent in England in 1036. Regrettably ambition outran brotherly loyalties and in the following year, having disagreed with Emma over her sharing rule with him, and with his own mother's backing, Harold laid claim to the kingly title. He ruled for three years, dying in 1040, when his brother was on the point of invading. He was buried at Westminster but Harthacanute obviously felt that this wasn't the proper place for him. The body was exhumed and thrown into a stagnant fen.
His reign seems to have been a time of tumult and of violence: certainly one of his half-brothers, Alfred, one of Emma's sons by Aethelred, was killed when he came to England from Normandy, at about the time Harefoot was negotiating his election as king. Emma's other son by Aethelred was Edward, who was to become Harthacanute's heir, and king of England in turn.