I think Tooth Fairy would be the first. She is very kind - hearted and is always interested in new things. Elsa's unique ice powers and her beautiful ice creations would draw Tooth Fairy's attention, and she would be eager to make friends with Elsa.
It could be Sandy. Since he communicates through dreams, he might enter Elsa's dreams first. In her dreams, he could show her some positive and calming images, which would make Elsa feel comfortable around him. And then, when they meet in person, Elsa would already have a sense of familiarity with Sandy and they could become friends quickly.
Black Widow could be the first to form a friendship with Elsa. Black Widow is very perceptive, and she would quickly realize that Elsa is not a threat but a potential ally. She has a way of getting close to people and making them feel comfortable. She could help Elsa adjust to the new and strange world of the Avengers, and they could bond over their shared experiences of being different from the rest of the group. Elsa's powers would also be something that Black Widow could see as a valuable asset in their missions.
Harry Potter himself would be likely to interact with Elsa. Given his own experiences with unique powers and his role as a hero, he would be sympathetic to Elsa's situation. They could bond over their shared sense of responsibility and their ability to do things that others can't. Harry could also introduce Elsa to his friends and the world of Hogwarts.
The most interesting pair could be Elsa and Bunny. Imagine Elsa's elegance and Bunny's gruffness. Bunny could teach Elsa about the magic of the Easter eggs and how they represent new life, while Elsa could show Bunny how she can create beautiful ice art. Their different personalities and powers would make for an exciting dynamic.
I think Jack Frost from 'Rise of the Guardians' and Robin from 'Young Justice' would be likely to form an alliance first. Jack is always looking for adventure and a chance to prove himself, and Robin is very strategic and open - minded. They would probably recognize each other's skills quickly and decide to work together.
In a fanfiction where they meet, Elsa could learn a lot from the Guardians. For example, Bunny might teach her about how to better control her powers in a more natural way, as he has a connection with nature. Elsa, in turn, could show them how to build an ice castle in a more efficient and beautiful way. Also, Sandy could communicate with Elsa through dreams and help her overcome some of her fears related to her powers, which could lead to a great friendship between them.
The trolls could play an important role. They are mysterious and have their own magic. In dark Elsa fanfiction, they might be the ones who try to either contain Elsa's dark powers or guide her towards using them in a more positive way. Also, a new character could be introduced, like a powerful sorcerer from a far - off land who is either drawn to Elsa's powers or sees her as a threat and wants to eliminate her.
In this fanfiction, Elsa might use her ice powers to create amazing ice sculptures for the Guardians. They could be in awe of her abilities and she could learn some new things from them too, like how to better protect the ones she loves.
Hermione could be quite popular. In the fanfictions, she might be the one who tries to figure out the science or magic behind Elsa's powers. Another could be Ron, who might be initially skeptical of Elsa but then grows to be her friend. Olaf from Elsa's world could also be popular as he brings his unique brand of humor to the mix when he enters the wizarding world.
It could be a power - hungry villain. Someone who sees Elsa's powers as a means to gain more control. Maybe a dark wizard or a warlord.
Anna would be a likely candidate to rescue Elsa. She loves her sister deeply and has shown great courage in the past. She would stop at nothing to find and free Elsa from Hans.