According to medieval legend, Kenelm (also spelled Cynehelm) inherited the throne of Mercia at the tender age of seven upon the death of his father, King Kenulf. His sister, Quendreda (also known variously as Cynefrith or Quoenthryth) coveted the throne for herself and so bribed Kenelm's tutor, a man named Askbert to kill him. This the tutor did while he and his charge were hunting in the forest of Clent. When his tutor took out his sword, the prince apparently responded that this was not the place ordained for him to die before putting his walking stick into the ground. The stick turned into a thorn tree, but this relatively useless miracle could not save the young king's life; Askbert was apparently unimpressed and decapitated him anyway. A dove flew out of the boy's wound and traveled to Rome, where it delivered a note to the pope which read,
"In Clent cow-pasture under a thorn;
Of head bereft lies Kenelm, king-born."
The pope then sent searchers to England, where they were guided to the body by a white cow. When they picked up the body, a holy light shone and healing water sprang from the ground. In fact, healing water sprang from the ground whenever they put the body down, which makes this author wonder why they would have wanted to reinter it. However, that is what they wanted to do, so they returned the body to Winchcombe Abbey. When they brought it by the princess Quendreda, she reportedly said that she had not harmed her brother and hoped that she would go blind if she did, whereupon her eyes promptly fell out and the monks declared Kenelm a saint.
It has since been shown by historians that Kenelm lived into adolescence for sure and that he may have even died before his father. He may have been killed in battle, although it is difficult to say anything for certain about him. However, the factual accuracy of the legend is irrelevant to the idea of Kenelm the saint since its main importance lies not in the events themselves but in the continuous retelling and belief in those events without regard to their objective truth or lack thereof.
In art, Kenelm is usually depicted as a young prince holding a blossoming rod and may be accompanied by a dove with a letter in its mouth. He is also occasionally shown as a boy king trampling the aforementioned sister. He was highly venerated throughout the middle ages in England, although he is now mostly honored at Gloucester and Winchcombe, the latter of which holds his relics and coffin. Kenelm probably died near 821 C.E., although information remains uncertain. His feastday is July 17.
Sources: http://users.erols.com/saintpat/ss/0717.htm, http://www.catholic-forum.com/saints/saintk.htm, http://web.ukonline.co.uk/conker/conkers-and-ghosts/kenelm.htm