One day the 21st Phantom was out near the riverside, hunting a man-eater leopard. He found and killed the great cat just as it was about to attack an old missionary.
However, the missionary and his wife were already dead because of malaria. But a baby boy, which the couple were going to adopt, was found alive. The Phantom tried to trace the boy's roots but couldn't. So he adopted the baby himself and named him Rex King ('twice a king').
The boy grew up to be a bubbly teenager, spoilt by the Phantom's love who was experiencing fatherhood for the first time (he hadn't married yet). Even now, when the Phantom has his own children, he hasn't let that lessen his love for Rex in any way.
Eventually the Phantom was able to trace Rex's origins: Rex was the lost prince of the kingdom of Baronkhan. Rex now rules there as king.