Obviously, Percy would be one of the main characters. He brings his own set of skills and experiences from his world. And of course, the Avengers. Maybe Tony Stark would play a major role as he is often the one to analyze new situations and technologies. Thor could also be important as he has some knowledge of magic and other realms which might help in understanding Percy and the 'atr' aspect.
The main characters could be Percy, Captain America, and Clint Barton. Captain America would bring his leadership and sense of justice. He might be the one to try to keep the peace and order when Percy arrives and the panic starts. Clint Barton, with his sharp eyes and combat skills, could be on the front line when it comes to dealing with any potential threats that Percy's presence might bring. And Percy himself, with his unique powers and abilities, would be at the center of the story.
Percy and Natasha Romanoff could be central characters. Natasha is great at assessing people and situations. She might be the first to try to figure out if Percy is a threat or not. Percy, on the other hand, with his quick thinking and combat skills from his demigod training, could team up with Natasha in some interesting ways. Also, Bruce Banner could be involved. His scientific mind might be crucial in understanding any strange 'atr' related things that are going on.
It could be that Percy is being chased by some powerful enemy from his world and he stumbles into the Avengers' territory. His sudden appearance and the panic that follows draw the Avengers' attention. The 'atr' might be related to a special power or ability that Percy discovers in this new situation. This new power could be the key to defeating his pursuers, but he has to convince the Avengers to trust him first.
Obviously, Percy would be the main character as it's about him having a panic attack. His friends like Annabeth and Grover could also be important characters. Annabeth might be the one who tries to use her intelligence to figure out what's wrong with Percy and how to help him. Grover could use his empathetic nature to comfort Percy during the panic attack.
A new, original villain could also be the main antagonist. This villain could be someone from Percy's past who has a grudge against him and wants to use the Avengers as a way to get revenge. By making Percy betray the Avengers, this villain can not only harm Percy but also cause chaos among the powerful superhero group.
The Rose family's eldest son, James Rose, could be a central character. He's a bit of a rebel and his recklessness might get the family into trouble or it could be the key to their survival during the vampire panic. Another character could be the family's matriarch, Mrs. Rose. She has a kind heart but also a firm will and she uses her wisdom to guide the family. And there could be a mysterious stranger who shows up during the panic and has a connection to the vampires but also becomes an ally to the Rose family.
Definitely Iron Man's protege. He could be a young genius who inherits some of Iron Man's technology and tries to lead the little Avengers. Then there's Spider - Man. His youth and agility make him a natural fit for the team. And of course, Ant - Man's daughter. She already has experience with shrinking technology and could bring a unique perspective to the group.
There could be a character like Nova, a young hero with cosmic powers. He could bring a fresh perspective to the team. Another could be Kate Bishop, Hawkeye's protege. She has great archery skills and a sharp mind. And of course, Miles Morales could be part of it too, with his unique spider - powers.
The main characters would obviously be the five Avengers. It could be Iron Man, Captain America, Thor, Black Widow, and Hulk. And of course, the baby would be a central character as well. Maybe the story could focus on how each of the Avengers interacts with the baby differently. For example, Iron Man might use his technology to create baby - friendly gadgets, while Thor might tell the baby stories of Asgard.
Obviously, the Avengers themselves would be main characters. Iron Man could be the one who funds the high - tech medical equipment in the hospital. Thor might have a hard time adjusting to the human medical procedures. And Black Widow could be using her espionage skills to find out if there are any threats in the hospital.
Annabeth would be a likely helper. She's very perceptive and has been through a lot with Percy. She would probably be able to sense something was wrong and offer her calm and rational presence to help him through it. Grover could also be there. He's Percy's loyal friend and his empathy for Percy's situation would make him a great support. He might even use his nature - related powers to create a more soothing environment for Percy.
To make it interesting, you can focus on the contrast between Percy's world and the Avengers'. For example, Percy's magic from the Greek gods is very different from the technology and superpowers in the Avengers' universe. When a panic attack occurs, this contrast can be highlighted. Maybe Percy's way of dealing with the panic attack is through some ancient Greek ritual, which the Avengers find strange but effective.