First off is Regulus Black. Obviously, these two must interact.
Second is Edwina "Eddie" Lovegood. She's a conspiracy theorist, bookworm, punk/rocker and highly intelligent (despite her many oddities). Her marriage to Xenophilius is fresh and she spends her time inventing potions and spells or tutoring students in wandwork, spells or potions. I can't think of an excuse for interaction at this point, but we must give them one.