This here is Jasper Nott. He's a '48 Slytherin, a Death Eater and a Healer in the Creature-Induced Injuries Ward. Most people would know him as a polite, urbane, slightly austere sort of man - an excellent healer with a good bedside manner who genuinely cares about his patients. Perhaps a bit conservative in some of his views but nothing outwardly harmful. He may even be viewed as being a bit eccentric with his interest in Muggle surgical techniques.
Inwardly, he's a different story. He's deeply dedicated to purism and the Death Eater cause and he truly believes in the superiority of witches and wizards over Muggles. He doesn't want to kill all the Muggles - nothing so gauche - but he does believe that they are inherently inferior and thus that Muggleborns possess that same inferiority.
What else...? Uh, he has a son, Mikael, who is open for play. His wife... er... died many years ago. And he is good friends with Melisande Yaxley. *nudge, nudge, wink, wink, say no more, eh?* Anyway, you can read all about him here and I am all up for family, friend, colleagues etc!
Oh, forgot to add - Jasper acts as a healer to the Death Eaters and their allies so he's on call for that if any of your characters ever need it.