Often, they interact with a sense of mutual wariness. Vampires, being undead and having their own set of powers, might be cautious around devils, who are often depicted as having more sinister and powerful magic. Sometimes they fight over territory or resources in the fanfiction world.
There are those where devils and vampires form an alliance to fight against a common enemy, like a powerful witch or a group of hunters. In these stories, their different powers and natures are explored as they work together. For example, the devils might use their magic to create distractions while the vampires use their speed and strength in combat.
Tonks' Metamorphmagus ability could be used in interesting ways in relation to vampires. For instance, she could change her appearance to look like a vampire to infiltrate a vampire lair. Harry's magic skills, like his Patronus charm, could be used against vampires if they prove to be a threat. Vampires' speed and strength could pose a great challenge to both Tonks and Harry, but their combined powers of magic and shape - shifting could give them an edge in a fight.
Fleur could use her Veela charm to try and disarm or distract vampires. Since Veela have a certain allure, it might work on vampires as well.
Well, in a lot of fanfiction, Naruto and Buttercup start off as strangers from different worlds. But as the story progresses, they start to respect each other. Naruto's determination might inspire Buttercup, and Buttercup's no - nonsense approach could make Naruto more straightforward. They might also team up to protect the innocent in a world that is a mix of both of their universes, with Naruto using his jutsus and Buttercup using her punches and flight to fight off bad guys.
In fanfiction, they often start as adversaries. Sylar being the power - hungry villain and Peter as the hero. But then their interactions can change. They might have intense power battles where they test each other's limits.
Often, they interact with a sense of mutual respect. Naruto, being the friendly and open - minded ninja he is, would approach Karai with an open heart. Karai, on the other hand, might be more guarded at first but slowly warm up to Naruto. For example, in a fanfiction where they are in a battle tournament together, they might start as opponents but then find common ground and help each other out.
In some fanfiction, Voldemort and Snape interact in a very master - servant kind of way. Voldemort gives orders and Snape follows, but with a hidden agenda. For example, Snape might be subtly changing the Dark Lord's plans without him noticing.
Some fanfictions depict Naruto and Mito interacting in a time - travel scenario. Naruto goes back in time and meets Mito. They interact as comrades, with Mito seeing Naruto as a strange but powerful ally. She might initially be skeptical of him but then comes to trust him as they face common threats together.
Often, they start off as adversaries. For example, in many fanfictions, Snape's dark and brooding nature clashes with McGonagall's no - nonsense attitude. But as the story progresses, they might find common ground in protecting the students or in the face of a greater threat to Hogwarts.
In some fanfiction, their interaction can be more of a mentor - mentee relationship. Diantha could be teaching Ash about the more advanced aspects of Pokémon battles, like strategic planning and reading opponents' moves. Their conversations would be filled with Pokémon knowledge and advice.