Someone who doesn't say "hey you, yes, you, you've been improving your techniques lately..." because he can't even remember your name or the names of other newbies.
In short, someone who knows each of his students' names from day 1, approachable, great skills and teaching ability, and observes and stops you and your partner during drilling to correct any bad habit that he sees from a distance. Also be able to determine his student's physical limitation and capabilities.