Walt Stone is one of Sadie Kane's love interests (other one being Anubis) in The Kane Chronicles book series.
He is a 16-year-old magician who was originally dying from a curse due to his ancestor a pharaoh named Akhenaton wish to destroy the worship of the Egyptian Gods and to worship the sun instead. So priests cursed Akhenaton and his descendants to die at a young age (which included Akhenaton's son Tutankhamen). As practising Magic tends to speed up the curse, Walt uses amusements instead which store Magic in advance. Walt's father had died from the curse when he was a teenager, just before Walt's birth so Walt's mother was worried about Walt as he was nearly sixteen and no doctors could find a cure to save him and it seemed Walt would die soon.
However, this was prevented when Sadie's other love interest Anubis, the God of Death merged with Walt in order to keep him from dying from the curse and so to save Sadie from having to choose between them.