One who can communicate effectively, is always in his/her agreed upon position, is never more than a slight turn of the head out of sight, can assist me if necessary under any conditions, keeps his/her cool during those times, has buoyancy control down, and knows that I will do the same for them. And along with all of this they need to be honest and have good judgment, including that which allows them to say I'm not doing this dive. I only have few people I dive with who meet all these criteria. All the students I've trained are among them allowing for the fact that some of them would not be doing certain dives with me(I'm not taking an OW diver on one of my deep wreck dives in 38 degree water or under the ice). The others I trust in all these areas have some technical training or other advanced training(GUE).