So, my upfront answer is yes absolutely, because we are firmly in the realm of fiction here, and you can make your story work how you want. But, I think it’s much more interesting to talk about what would actually be required to make this work:
Your person and your demon have to be physically sexually compatible. In layman’s terms, this means that all the important bits are going to have to fit together, and you might be laughing, but this is actually one of the biggest barriers to ensure that two incompatible species don’t mate.
They also have to be genetically compatible. Humans have 23 pairs of chromosomes that contain all of our DNA. If your demon doesn’t have the right number to pair up, then the process won’t go. Sometimes, in other hybrids, you can get away with the wrong number (this is the case in mules, who have an extra, unpaired chromosome), but this is fairly rare, and it does mean the hybrid is sterile. There also have to be matches for each gene. We require two copies of every gene (hence why each chromosome needs a buddy), and only being able to express the single available copy would be quite disastrous.
The conditions for growth have to be met. In this case, if your demon is really physically hot or something along those lines, and your hybrid baby requires human womb temperatures, that’s going to cause that embryo to fail as well.
I hope this helps! There would probably be some other conditions to meet, but these would be the absolute basics. Happy writing!
The conditions go much farther than that, and really chromosomal homology does as well, but for the purposes of fiction much of that is irrelevant. That also simply makes it less likely, and doesn’t affect whether it’s possible or not.
What I think would be really interesting is to have a half-demon child that resembles neither parent. That would be because the epigenetic factors, which are themselves controlled as much by ontogeny as genetics, could potentially get all kinds of messed up. Even if the parents produce a viable offspring, that doesn’t mean the conditions in the womb are ideal for that fetus, or that they won’t have some profound impact on the child’s development.
At the very least, it’s an interesting rabbit hole to explore. 🙂
Precedent exists for human/demon hybrids, but all the examples I can think of are demon fathers and human mothers. I imagine this (and the fact that most demigods have a divine father and human mother) is because you know the slightly odd baby’s mother is human because it came out of her, but the father could be a mystery.
Also, this comes at a serendipitous time, as I am working on the origins of Lemmaverse demons and how they make more demons.