A 13th Cylon

I didn't think about it much the first time I watched this episode of Battlestar Galactica but there is no way the writers just dropped in that little bit of information for no reason. A 13th cylon, who was evidently an artist and killed by Al.(Brother Cavill)
This could be a way to make the rebel Cylons more acceptable to the viewer, making one of the others the real bad guy who led them all astray but I suspect it isn't, but if there is another Cylon who would it be.
I think the answer is simple, it's got to be Boxy. Why else would they have put this character in the mini-series except as a way to surprise us when he all returns in the finally to solve the entire problem. the shows writers have said that they would end the show with a bang and I for one can't come up with a more surprising conclusion than if Boxy, probably along with Muffy, the robot dog to save both the Cylons and the humans, except if they were able to use their time traveling motorcycles to go back in time on earth and stop the atomic war, but then the Cylons wouldn't come to the colonies and create the human models and... O.K. no time travel this time.