First, my an definition of the term "having (or covering) someone's back." This is not simply someone to help in a fight, though they might be called upon to do that as well. Covering someone's back means that they are more concerned with that person's protection and well being than the actual fight. Anyone or anything that manages to get to you has to deal with them first.
Most definitely NOT either Buffy or Angel, if I was wanting someone to watch my back and, most importantly, follow my orders. Both are good leaders, but neither one is very good at doing what they're told, When things get really serious, the person who you count on to cover your back has to be relied on to do exactly that, not to try and second guess you or go off on their own tangent.
As for Faith, she tends to be impetuous, rushing headlong into whatever comes up, kicking and punching in all directions, relying on main force to get the job done. Someone whose job is covering your back has to be more concerned with that than actually getting into the fight itself.
Illyria is a hell god, almost certainly not used to following anyone else's orders, or holding back when the action starts. She would be a great general, or a real asset to anyone's fighting force, but couldn't be relied upon to be what amounts to a bodyguard.
Spike would be ideal for the job if he were as devoted and loyal to me as he was to Buffy, but he has one fatal flaw. He loves to fight. He couldn't be relied on to not go Viking berserker at the worst possible moment. Secondary to actually protecting you from attack from the rear, the person guarding your back has to be relied in to protect and get you out of harm's way if the worst happens and you do get injured. Spike just isn't the type to run away from a battle, even if his reason in diong so is to get his fallen leader to safety.
Connor? Maybe in another twenty or thirty years, if he gets enough experience and self discipline. Not now.
The person who I would pick to cover my back would have to be someone with a strong sense of duty, committed to following the orders of his leader without question. A reluctant warrior, at best, and someone who realizes that victory isn't always the prime goal. The person who fights and runs away, who will live to fight another day. Someone who might admire heroes, even envy them, but who has no real desire to be a dead hero when it's possible to be a live coward.
When the going gets rough, and the rough get going, Andrew's my man, all the way.
