I think he is a main character now because he survived the prison. I kind of like him, was ticked at him for taking the alcohol, or what ever it was, from the vet school, but that seemed realistic. He is shown to be a flawed character with believable weakness, we would all deal with the ZA in our own way and for an alcoholic it would likely be finding a drink.
It's also nice to have someone along who doesn't have to be center stage, he is not going to take over or be a threat to anyone and you need some filler, besides we have to keep finding new people sense they seem hell bent on killing all the old ones. I think we might just being seeing him in a different light now because there are only the three of them so he might feel more comfortable in showing his personality, rather than being reserved. Unless he was the cause of the deaths of the other groups I don't see that as an issue, even our group has lost a lot of people, it happens at the end of the world.
That's what I was going to say to the question of when did he become a main character? When he survived the prison.