The King of Hearts, like the Queen of Hearts, was one of the deadly pawns used by the Celestial Toymaker against Steven Taylor and Dodo Chaplet when they were trapped in his realm. The Queen referred to him by the name Henry. When not in a contest, the King stayed in playing card form. (TV: The Celestial Toymaker)
Behind the scenes
The King of Hearts appears like a living playing card. A living King of Hearts also appears in Alice in Wonderland.