The gaang's reaction could be mixed. Zuko would be extremely emotional, as Ursa is his mother. He might be overjoyed and at the same time a bit angry for all the years they were apart. The others, like Toph, might be a bit more cautious. Toph would probably sense Ursa's emotions and try to figure out if she can be trusted. And Sokka and Katara would be observing the whole situation closely, waiting to see how it unfolds before fully engaging with Ursa.
Some of the gaang members might be very excited. Aang, being the Avatar, would probably be curious to learn from Ursa. He could ask her about the Fire Nation's history and how it relates to the balance of the world.
In the fanfiction, it could be a very emotional encounter. Ursa might be shocked to see how much her son has grown within the gaang. She could share her past experiences and wisdom with them, which would help the gaang in their future adventures.
Some of them might be excited. They could see new ideas and interpretations of their world that they hadn't thought of before. For instance, if there are new bending techniques in the fanfiction, the benders in the gaang might be eager to try them out in their imagination. Others might be more critical and point out inaccuracies compared to their real - life experiences in the Avatar world.
They'd probably be shocked. Zuko's past is full of hardships and they might not have expected all that he's been through.
They might be in awe. Gallantmon is a very powerful and majestic digimon. They would probably be a bit intimidated at first, just looking at his large and imposing form.
Luke might be in awe. He's always been seeking his family and a sense of belonging, so meeting his parents could make him feel complete. He'd probably listen intently to whatever they have to say.
Some of the Avengers, like Thor, might have a more complex reaction. Since he is Loki's brother, he may feel a sense of responsibility towards the children. Others like Captain America would be cautious but fair, trying to assess if they are a threat. Iron Man, on the other hand, would probably be sarcastic and make some snarky remarks while sizing them up.
They might be cautious. After all, they don't know her and she's from Percy's world which could be full of unknowns for them.
They would probably be very concerned. Jungkook might be the first to approach her and offer his help. They'd do whatever they could to make her feel better, like giving her a warm hug.
They would probably be shocked. Demigods have extraordinary abilities, like being able to control water or run super fast. Percy's mortal friends, who are used to a normal world, would be taken aback by such powers.
Thor might be conflicted. He'd see his nephew/niece but also remember Loki's treachery. Iron Man would probably be sarcastic at first, making remarks about the family genes. Captain America would be more cautious and diplomatic, wanting to give her a chance but also being on guard. Black Widow would try to read her body language and motives right away.