In some fanfictions, it could be a powerful angel. Since Lucifer was an archangel, an equally powerful angelic being might be seen as his mate, perhaps one with a unique connection to him from before his fall.
The Lucifer comic was written by a team that includes Neil Gaiman and others. Neil Gaiman's contribution was significant in shaping the initial storylines and characters.
Obviously, Harry is one of the main characters as he is the vessel. And Lucifer, of course, is a key figure as his essence is involved with Harry. There might also be other characters from the original Harry Potter universe, like Hermione and Ron, who could be trying to deal with the situation of Harry being Lucifer's vessel.
The main character would be the daughter of Lucifer. Depending on the plot, other important characters could include some of the powerful demons who might be trying to control or use her. Also, there might be some human characters who get involved with her story, perhaps a hunter who initially wants to kill her but then realizes she's not all bad. And of course, Lucifer himself might make appearances to influence his daughter's actions.
Another theme is the power dynamics. Given Lucifer's status as a fallen angel and his powers, how does his relationship with his wife (the OC) affect or change the power balance? For example, does she gain some powers through their connection? Or does she help him control his more devilish tendencies?
Lucifer in the 'Never Ending Story' might be a character that represents a form of antagonist or a source of chaos. It could be a being that stands in opposition to the main protagonists' goals. Maybe it has some sort of supernatural powers that make it a formidable force within the story's universe. But again, this is all speculation without more in - depth study of the story.
Chloe Decker is also popular. In fanfiction, her relationship with Lucifer can be delved into more deeply. Writers might explore different scenarios of their relationship, whether it's a more romantic angle or a platonic partnership where they solve mysteries together in ways that the show might not have fully explored.
Sam Winchester is quite popular to pair with Lucifer. Their connection in the show, with Sam being one of Lucifer's vessels, makes for an interesting dynamic in fanfiction. Also, Castiel. The angel's moral compass and his power contrast well with Lucifer's nature.