Fictitious characters:
This relies on some assumptions about the domain of possible worlds. Fictitious stories are mostly fictitious objects, thus we must know whether the framework of possible worlds has a constant domain (that is, a single domain to all worlds) or a variable domain (domains are attributed to each world by a domain function). The endorsement of the conjunction of both Barcan formulas imply constant domain (it is a simple result from modal model theory, as can be seen in Fitting's book on first order modal logic). We must, therefore, not endorse them if we want to have fictitious objects as merely possible objects*. If we have them as merely possible objects, we may assume that there is a class of possible worlds** that are consistent with the narrative constructed by the author.
We may as well not endorse fictitious objects as possible objects, but as actual objects that are a product of our intensional labor (the kripkean approach of artifacts). They would exist independently of our ontology of possible worlds and our modal realism would turn out innocuous.

As for the existence of worlds: it is implied by modal realism.

*Possible objects that are not actual.
** Since fictitious narratives are not exaustive or maximal relative to their fictitious realities.