She might be very surprised at first. Given Jon's unique situation, like being a Stark but also having ties to the Night's Watch. Brienne would then start to assess his character and might be quickly won over by his noble qualities.
In the fanfiction world, Brienne's reaction could be complex. She has her own code of honor and set of beliefs. When she discovers Jon, she'll likely compare him to other leaders she has known. If he passes her internal 'test', she could become one of his most ardent supporters. She might also be protective of him, using her skills as a warrior to keep him safe from his enemies. This would be especially true if she sees Jon as a beacon of hope for the future of Westeros, a place she loves despite its many flaws.
In some fanfictions, Missandei may be initially distrustful. She has seen many power - hungry people in her life, and Jon's sudden appearance and his potential claim could make her suspect his motives. However, as she gets to know him better, her opinion might change.
In certain fanfiction scenarios, Brienne's discovery about Jon could lead to an interesting dynamic. Brienne is a character who values integrity above all. When she realizes who Jon is and what he stands for, she may feel a kinship with him. They both have faced hardships and prejudice in their own ways. She might view Jon as a glimmer of hope for a better Westeros. She could become his champion, not just in battle but also in the political arena, using her influence to gather support for Jon's leadership. This could also lead to some great character interactions and development for both of them as they navigate the complex world of Westeros politics and warfare together.
In some fanfictions, she might be initially shocked. She could see Jon as a potential threat to Daenerys' claim to the Iron Throne, given his true parentage. This discovery could lead to her trying to gather more information about Jon in secret.
She might be shocked and feel a great deal of sympathy for Percy. She could immediately try to comfort him and assure him that he's safe now.
Ginny's reaction could also involve a sense of helplessness at first. But then she'd quickly gather herself and use her resources. For instance, she might call on her friends and family for help. She'd be by Harry's side constantly, using her presence to comfort him and also using her knowledge of magic to try and ease his pain.
She might be overjoyed. After all, Naruto is her son. She could immediately rush to find him and embrace him tightly, tears of happiness streaming down her face.
In other cases, Teela's reaction could be a mix of emotions. She could be happy that Adam is such a powerful hero but also a bit sad that he felt the need to keep it a secret from her. This discovery might make her question their entire relationship up to that point, and she may need time to sort out her feelings about it all.
She's usually shocked and conflicted. It's a huge moment for her character.
He might be shocked. Fanfiction can often be quite different from the original source material, and the nogitsune concept is already pretty out - there. So the shock value could be high.
In many fanfictions, Naruto's reaction is a mix of shock and determination. The shock comes from finally learning about his clan, and the determination is to uphold the Uzumaki name. He could start wearing something that represents the Uzumaki clan, like a symbol on his clothing. He also may become more protective of the Uzumaki secrets and legacies he discovers.