It depends on the specific content of the fanfic. If it adheres to general guidelines of no excessive violence, inappropriate language or adult themes, it could be suitable for a wide range of ages. But without actually reading it, it's hard to say for sure.
Naruto himself could be an interesting pairing with Lady Death. His positive and determined nature could contrast well with her more somber and death - related aspects. Another could be Sasuke. His dark past and brooding personality might have an interesting interaction with Lady Death. And Kakashi could also be a good choice. His experience and calm demeanor might make for an engaging relationship with Lady Death as they face various challenges in the Naruto universe.
Lady Death Lemon could be an original character created for this fanfiction. She might be a powerful being with a connection to death or the afterlife in some way, perhaps having powers related to life - force or the passage of souls, which would be really interesting in the context of the Harry Potter universe.
Generally speaking, you won't find nudity in Lady Death comics. The comic's appeal lies in its plot and artistic style rather than resorting to nudity for effect.
The main characters would obviously be Ben 10 and Mount Lady. Ben 10, with his Omnitrix that allows him to transform into various alien forms, brings his unique set of powers and a sense of adventure. Mount Lady, on the other hand, with her ability to grow to a large size, provides a different kind of power dynamic. She is also part of a hero community in her own world, which can add interesting elements to the fanfic as their two worlds collide.
I'm not sure specifically which 'Mount Eerie real death story' you're referring to. There could be many incidents or tales associated with a place named Mount Eerie. It could be a climbing accident, a natural disaster - like an avalanche or a landslide - that led to deaths. Without more context, it's hard to say exactly what the story is.