Well, here's the thing. I have to agree with Walker, if I understand the above right, that essentially the definition of a "god" rests in their relationship with humans; which is to say, there's no fundamental difference between a god and any other spirit beyond the way humans interact and perceive that spirit. With or without that spirit's active encouragement.
But I also think, again, just as my own opinion from my own experiences, that spirits in general don't often get a lot of credit from us humans for being at least as smart, adaptable, and as much individual people as we are. If a spirit has hung around long enough to be a god and made it this far, I'm betting that spirit is quite capable of defining and presenting themselves in more modern forms that are more understandable to modern people. So, are fictional characters real spirits? I write fiction, and I will say, there's a strong difference between writing a character and working with a spirit. On the other hand, I've written novels with characters that were strongly based off of the initial dream contact a spirit made with me. In the same breath, if a fictional character is more accessible to a modern human a spirit wants a relationship with, I don't have any doubt that spirit will present itself in that form. (And probably, not only gods. In the parts of the world where ancestor veneration is rejected, I imagine a creative and determined ancestor might do the same thing.) Frankly, that's where "real" not only gets tricky, but is usually badly defined, especially in occult work.
I will differ with Walker regarding the "reality" of fictional worlds. In my experience, they're not simply "real" for their writers, they are very much "real" for their readers/watchers - sometimes more so, the sane writer draws lines in their minds and lives differentiating their fact from their fantasy, and audience often doesn't. Slenderman was already in the realm of possibility the moment it was created and released, though certainly the feeding of spiritual energy must have made him stronger. But the very answer to your question, I suspect, as to why Slenderman instead of PeppaPig lies in the power of the storytelling around him. Storytelling is an INCREDIABLY powerful occult tool.