Q: How do I mark someone as important while using "do not disturb"?

Add them to your Favorites in the Phone app. then set Do Not Disturb to Allow calls from favorites.

http://support.apple.com/kb/HT5463

You also have the option to allow persistant callers (Multiple calls in a very short time) to ring thru.  Rusher2004 is correct, but the other option would work as well.