Kos could be a leader but he's having his legs chopped out from under him by HeWhoShallNotBeNamed.
Kos is our Alert 5, Flying Squad, International Rescue defender. We use his pace to rush him to the defensive hot spots as they flare up and more often than not he gets there in time. Unfortunately for him he almost always at least makes it into the vicinity of the crisis and as such offers himself up for blame when it all goes to shit. I think Bellerin is a great young player but he's not a patch on, for example, Ashley Cole in terms of his defensive game. Cole by the same age was a much better player. I haven't noticed Bellerin improving at a steady rate and a lot of this is masked by the fact he's such a potent offensive weapon.
Monreal is a better defender but frustrating going forward and he's crippled by his lack of pace, particularly as he plays in a team that defensively relies on pace rather than teamwork and planning.
Mustafi and Gabriel. Don't know. Sometimes good, sometimes suspect. Gabriel to a degree can cover for Kos because he has that tear about element to his game that's required to be a "good" defender in Wenger's crackpot system. Mustafi seems to be a bit more level headed and that probably works against him. He needs to be a lot more disorganised and random if he wants to do well here.
Holding and Chambers were evidently trained elsewhere in the dark arts of traditional defending so they are out of their depth at Arsenal.
Can any of them be leaders in such a haphazard non-system? They are too busy just trying to grip on to their own task to worry about the chaos around them I would think. Maybe Cech is the guy who needs to step up a bit more but even he's suffering the Wenger effect it seems.
There's only one leader at Arsenal. Alexis will learn that too, if he sticks around long enough.