Don't read too much into it. He's the most important non-Good character that is the same size as Belkar, thus keeping the composition of the poster balanced. The fact that he also represents a group that is not associated with either Xykon or the Linear Guild helps.
Really, it was down to either him or the black dragon mother for that top slot, and the dragon is too big and would have blended in with the
(and is already dead).