She could be a new student at Hogwarts. Maybe she's a transfer from a different magical school in another country. This would be an interesting way to introduce her character as she'd have different knowledge and experiences compared to the regular Hogwarts students.
One plot idea could be that Erin Reagan somehow gets transported into the Harry Potter universe. Maybe she has a special power that is recognized in the wizarding world and she has to help Harry and his friends in their fight against Voldemort. For example, she could have the power of super - fast communication which helps the Order of the Phoenix share information more quickly.
Sabine could be integrated as a foreign exchange student from a magical school in another part of the world. She could bring her own unique set of magical abilities and cultural beliefs related to magic. For example, if she has a different way of casting spells or making potions.
Aphrodite can be integrated by having her interfere with the love potions in the Harry Potter world. Since she is the goddess of love, she might have her own take on how love potions should work or might even try to create a more 'authentic' form of love magic. For instance, she could find the existing love potions too artificial and introduce a new kind of potion that is based on true love rather than just infatuation.
She could be a visitor from another dimension. Arya's skills with a blade and her ability to adapt would be interesting in the context of the magical world. She could form alliances with the characters, like Hermione, and they could learn from each other. For instance, Hermione could teach her magic, and Arya could teach Hermione self - defense without magic.
One way is to have the Harry Potter characters attend a concert of a band. For example, Harry, Ron, and Hermione could go to a wizarding world concert of a popular magical band.
One way is to make them magical steeds. They could be used by wizards for long - distance travel, faster than broomsticks. For example, a horse that can teleport short distances. Another way is to have them as part of Hogwarts' stables, where students learn to take care of them and ride them as part of a new class. Maybe there are wild horse - like creatures in the Forbidden Forest that have special powers and are a source of mystery for the students.
It could be introduced as a result of a failed experiment by a dark arts practitioner. For example, a Death Eater tries to create a powerful creature to use against Harry, but the chimera has its own will and doesn't want to be used for evil. So it escapes and finds its way to Harry, who then has to deal with this new and dangerous but also potentially helpful being.
The Dragon Girl can be integrated as a new student at Hogwarts. She could bring her own set of magical abilities related to dragons, like the ability to communicate with them or use dragon - inspired magic. This would make her stand out among the other students and lead to interesting interactions with the existing characters.
The Wendigo could be a creature that exists in a hidden part of the wizarding world, perhaps deep in the mountains near Hogwarts. Wizards who go astray might encounter it. For example, a group of students on an unauthorized adventure could stumble upon the Wendigo's lair.
They can be introduced as allies. For example, they could come to Hogwarts when they sense the dark forces rising. Their faery magic could be different from the regular wizarding magic, like being able to control nature in ways wizards can't. This would make them valuable in battles.