Actually, Sigma was created by Dr. Cain based off of X's design. Because X was the last of Dr. Light's creations, Sigma shouldn't exist yet, especially considering Dr. Cain found X's capsule well after the events of the final Megaman Classic game (chronologically).
@Hyper Shadow & @--Floob--: This is an alternate universe. Sigma is a human here, and he dislikes robots... mostly the ones that can function and plan by themselves... such as Mega and Dave and other Robot Masters. Another difference is here, Ciel, from the Mega Man Zero games, is the Mayor.
@--Floob--: Well not PERFECT sense, other characters could have been used, like... Signas, was it? The leader of the Maverick Hunters? But yeah, it makes sense in this dimension that he's alive in this time period. And human.
Yes, but Sigma was the leader of the Maverick Hunters before the events of X1. If it has to be a leader of the Hunters as police chief, Djoing could have used Sigma, Signas, Dr. Cain, or even Zero, since he was the Hunter leader during X1. Sigma probably just had the best sprites.
@--Floob--: Well, Zero will probably be one of Wily's creations alot further into the comic. Possibly. As for Sigma having the best sprites, good point... Sigma is the only choice there, apart from Zero, who had 8 bit sprites in Mega Man Xtreme (I'm not even joking.)