It would probably be tense. Nico is used to a world of danger and is a bit of a loner. Umbridge's officious manner would likely put him on edge immediately. He might sense her underlying nastiness right away. He would probably be very guarded in his responses to her, not wanting to give away too much about himself.
It could also be James. James is a natural leader and is always looking to include interesting people in his circle. When he sees Nico, he might sense that there's something special about him. He'd probably start by asking Nico about his abilities and before long, they'd be chatting like old friends. Plus, James has a big heart and would be accepting of Nico quite quickly.
Well, if Nico di Angelo meets Voldemort in a fanfiction, it depends on the writer's take. Nico could see Voldemort as just another evil entity to oppose. He might use his connection to the Underworld to his advantage. For example, he could summon some undead creatures to fight against Voldemort's Death Eaters. Voldemort, on the other hand, would probably view Nico as a strange and unexpected foe. He might try to use the Killing Curse on Nico, but Nico's ability to disappear into shadows could make it difficult for Voldemort to land a hit.
They would probably start with a bit of caution. Nico is used to his own world of demigods and Dumbledore to his wizarding world. But once they start talking, Nico might be curious about Dumbledore's magic, and Dumbledore about Nico's powers.
Nico might be cautious at first, trying to understand what kind of threat Voldemort poses. Voldemort would probably see Nico as an oddity and a potential obstacle to his plans, so he might attack without hesitation.
Another potential storyline is a power - struggle one. Nico is a powerful demigod, and the Cullens are strong vampires. They might initially compete to see who is more powerful. This could lead to a series of challenges and battles. However, as they fight, they start to respect each other's abilities and eventually come to an understanding. Maybe they even learn from each other's powers and develop new ways to use their own abilities.
A third plotline could be a more comical one. Nico, being from a different world of demigods, might not understand all the wizarding world's customs. His interactions with Umbridge could lead to some really awkward and funny situations. For example, he might accidentally use his powers in front of her in a way that completely baffles her, or he might misunderstand some of her rules and break them in the most unexpected ways.
Most likely, it would be Harry Potter and Nico di Angelo. Harry is the main character in his world and is always the one to step forward when something strange is going on. Nico, being the son of Hades, has a certain air of mystery around him. So, when he appears in the wizarding world, Harry would be the one to approach him out of curiosity and a sense of duty to protect Hogwarts from any potential threat.
Nico could be a valuable addition in terms of his unique powers. His ability to manipulate shadows and travel through them would give the Avengers an edge in stealth operations.
In a fanfiction where Nico di Angelo meets Dumbledore, there are numerous possibilities. Nico might be initially distrustful of Dumbledore, given his own experiences with authority figures. However, Dumbledore's calm and kind demeanor could gradually break down Nico's walls. Dumbledore could introduce Nico to the wonders of the wizarding world, such as spells and magical artifacts. Nico, in turn, could show Dumbledore some of his demigod abilities, like shadow - traveling. Their meeting could also lead to a cultural exchange of sorts, with Nico learning about Hogwarts and Dumbledore getting a peek into the life of a demigod.
In such a fanfiction, there could be an epic battle. Nico's powers from the Percy Jackson universe might clash with the Winchesters' hunting skills. Maybe they initially misunderstand each other, thinking the other is a threat. But then they could realize they are all fighting against some form of evil and team up. For example, Nico could use his ability to control the dead to help the Winchesters deal with a horde of zombies, while the Winchesters could teach Nico some of their combat tactics against demons.