Perhaps she is a time - traveler from the future. She has knowledge of future events that will endanger the ninja world and Sasuke. So, she tries to enlist his help, and in the process, a relationship develops between them. She might have unique abilities due to her time - traveling origin.
Perhaps it's a long - lost relative of one of the existing Order members. For example, a niece of Sirius Black who has just come into her magical powers and is eager to help the cause. She would bring a new dynamic with her connection to an established character.
She could be the daughter of a minor god that hasn't been introduced much in the original Percy Jackson series. For instance, the daughter of Hecate. This would give her powers related to magic, which would be really cool in the context of the story. She could use her magic to help Percy and his friends in battles or to solve some of the mysteries they encounter.
Sure is. 'The New Girl' doesn't rely on any prequels or sequels to make sense. It stands on its own, giving you a complete reading experience from start to finish.
There could be a friendly teacher. This teacher notices the new girl's struggle to adjust and offers her support and advice. For example, the teacher might give her tips on how to make friends or deal with schoolwork.
The new girl's character can be developed by giving her a unique backstory. Maybe she grew up in a different country and has a different style of martial arts. This would make her stand out and also influence her interactions with the other characters.
A character like a half - alien, half - human girl would be great. She could have the strength and some abilities from her alien side, but also the emotions and relatability of a human. This would make her a complex character with a lot of potential for growth and interesting interactions with the other Young Justice members.