Harry Potter x Hazbin Hotel/HaB
*Harry winds up in Purgatory due to his actions on Earth - taking revenge on those who wronged him. It isn't because of the revenge itself, but his maliciousness in enacting his revenge and some of what he did to people for revenge. While in Purgatory, he meets a certain female spider "demon" by the name of Molly. They become fast friends and Harry learns a lot about the afterlife; mostly where he is and why he's there. He also learns about Molly's family, mainly her brothers and that reside in Hell. Curiosity had always been one of Harry's better and worst traits.
(Harry/Molly, or Harry/RadioDust; which ever you're most comfortable with)
*Harry is the son of Lucifer and Lilith(going by a different name). Due to a ritual gone wrong, he is summoned to Earth and his reborn as a human wizard infant. He becomes the son of James and Lily. He is less than impressed. The fact he is actually a demon means he has near perfect memory as a human baby. He knows Dumbledore isn't to be trusted, because he remembers Dumbledore can't be trusted. Lies, betrayals, abuse, revenge, murder? All the fun stuff. Harry eventually gets back to Hell, most likely after suiciding or being murdered, and plots (more). He does agree with his sister (whom he missed while being trapped on Earth) that Hell can't keep going as it is, he's a little more real/cynical about humans and human sinners so makes his own suggestions about restructuring Hell and the Hotel.
*Lily summons and makes a deal with Alastor to get a son. Harry is that son.
*Alastor was a powerful wizard/voodoo practitioner in his human life, which is why he is so powerful as a demon. Harry is his descendant.
*Harry goes to Hell after being killed in the war, because he was a secret cannibal who ate his relatives and unlucky individuals from that wound up on market in Knockturn Ally. Once in Hell, Harry finds and uses Holy tools and machinery to perma-kill his victims (helping the overpopulation problem).
*Wizards (and many other - most if not all - magical creatures) are decedents of either angels, demons or both and their mingling with humans (and other creatures). Some creatures may have been created by either angels or demons to be used against the other and wound up mating with normal animals or humans which then went on to have offspring with each other or other creatures.
Both demons and angels being ethereal/spiritual, either for good or evil, they don't follow the same kind of rules in regard to genetics and biology that material creatures (humans, anything of Earth) do. So crossbreeding is very much possible regardless of the species in question.