Job titles and responsibilities vary greatly from one place to the next. The duties for a particular role at a particular place depend on what the two relevant parties have agreed upon. The particulars of that (such as authority of a manager to assign additional responsibilities, legal ramifications, and so forth) extend beyond the bounds of game development.

Game development teams often have overlapping roles and require a certain amount of fluidity, especially on small teams.

Both options you described sound reasonable. That is to say, designing characters for the comic content could reasonably be assigned to either role, or possibly even both.