Edward V

... was King of England from the death of his father Edward IV, on 9 April 1483, until 26 June of the same year. (I make that 78 days; Wikipedia says 86.) Edward was never crowned, and his reign was dominated by the influence of his uncle and Lord Protector, Richard, Duke of Gloucester, who succeeded him as Richard III. This was confirmed by the Act entitled Titulus Regius, by which Edward denounced any further claims through his father's heirs.

Along with his younger brother Richard of Shrewsbury, Duke of York, Edward was one of the so–called Princes in the Tower, who disappeared after being sent to heavily–guarded royal lodgings in the Tower of London. Responsibility for their deaths is widely attributed to Richard III, but the lack of any solid evidence and conflicting contemporary accounts also suggest other possible suspects.

Edward V was 12 years and 158 days old when he succeeded his father. Along with Edward VIII, and the disputed Matilda and Jane, he is one of four English monarchs since the Norman Conquest who were never crowned. It's generally assumed that he died (probably murdered) shortly after his disappearance, and this makes him the shortest–lived male monarch in English history.