I've always thought William IV was a rather interesting person, in part insofar as his long relationship (and 10 children) with an actress is concerned, and his apparent determination to survive until his niece Victoria was old enough not to need a regent. He couldn't stand her mother.
I've also always been fascinated by John of Gaunt, who wasn't a British king but did act more or less as regent for his nephew Richard II when the latter was still a minor. Gaunt maintained a long relationship with a woman most people would have considered unsuitable to become the wife of a prince, but he eventually married her. Together, through one of their illegitimate children, they became the direct ancestors of Henry VII Tudor and thus of the modern royals.