They might start as antagonists. For example, if a mercenary is hired by a dark wizard to capture Harry Potter, they would directly oppose the main character. But sometimes, they can also be reluctant allies. Maybe they are offered a large sum by Dumbledore to protect Hogwarts and end up working with Harry and his friends.
Mercenaries often interact with the main characters in a transactional way. If they are hired by the Order of Phoenix, they will do what they are paid for, like providing security or intelligence. However, they may not always be completely honest or upfront. Their loyalty can be bought, so the main characters, especially the more idealistic ones like Hermione, may be skeptical of them. There could also be situations where a mercenary has a change of heart during the course of the story and becomes more of a true friend to the main characters rather than just a hired hand.
They might initially be seen as a threat. For example, if the Cainite have a connection to dark magic, the main characters like Harry, Hermione, and Ron could be suspicious of them.
They might interact as allies. For instance, Itachi's combat skills and strategic mind could help Harry in battles against Voldemort's forces. Harry, in turn, could introduce Itachi to the magic of the wizarding world, like spells that could enhance Itachi's abilities further.
Often, Naruko and Harry might start as curious about each other's worlds. Naruko could be fascinated by magic wands and spells, while Harry might be intrigued by her chakra and ninja tools. They would exchange knowledge, like Naruko teaching Harry some basic ninja stealth moves, and Harry showing Naruko how to use a simple levitation charm.
Often, they interact with a lot of suspicion at first. Harry is used to a world of magic with rules and ethics, while Klaus is from a more cut - throat supernatural world. So, their first meetings usually involve sizing each other up. For instance, Klaus might see Harry as a potential pawn in his power games, while Harry would be cautious of Klaus's violent reputation.
They could interact in a way where Carlisle imparts his knowledge of self - control and morality to Harry. Given Carlisle's long existence as a vampire, he has a different view on life and death compared to Harry. This exchange of ideas could lead to some interesting character development for Harry. For instance, Carlisle might share stories of his past mistakes and how he overcame them, which could inspire Harry in his own battles against evil.
In some fanfictions, Harry Potter might be defiant towards Lucifer. He would use his Gryffindor courage to stand up to Lucifer's intimidation tactics. For example, when Lucifer tries to manipulate him, Harry would firmly say no and use his magic to create a protective shield.
Well, they might start off as enemies. The Stormtroopers, with their strict military discipline, could see the wizards as a threat due to their unpredictable magic. The wizards, on the other hand, might view the Stormtroopers as just another group of muggles with strange weapons.
Shifters might interact with existing characters in various ways. For example, they could be friends with Harry and help him in his battles against Voldemort. A shifter could turn into a snake to spy on the Death Eaters, providing valuable information to Harry and his friends.
As the story progresses, they start to collaborate. They use their unique skills to help each other out. Harry might use his magic to protect Rogue from certain threats, while Rogue could use her combat skills and mutant powers to assist Harry in battles. A lot of fanfictions explore how they learn from each other's strengths and weaknesses, like Harry learning about Rogue's experiences with being an outcast which he can relate to as a wizard in a Muggle - dominated world.
They might interact through dreams. A character could have a prophetic dream sent by a deity, guiding them on a certain path or warning them of danger. For example, Harry Potter himself might dream of a deity showing him a hidden chamber in Hogwarts that holds the key to defeating a new dark force.