Narcissa Malfoy

It's true that his imbroglio with Ganymede was more the aberration than the rule, but I cannot think he would have been either so careless or so insensitive as to entangle a witch in such a situation and then return with no word, no sign that he had left a family behind. There's the possibility he didn't know, of course, but that too sounds much more like his brother than Regulus himself.