Thor might accidentally start a conflict at first. His brash and confident nature could be misinterpreted by Percy and his friends. For example, if Thor makes a grand entrance and shows off his power, Percy's group might think he's being aggressive. This misunderstanding could lead to a brief moment of tension or even a small scuffle before they realize they're on the same side.
It could also be Ares, from Percy Jackson's side. Ares is the god of war and is often looking for a fight. When he sees the powerful Avengers, he might be eager to test his strength against them. He may not care about the potential consequences and just jump into a conflict, dragging others along with him whether they like it or not.
Loki could be the one most likely to start a conflict. He's always been a bit of a troublemaker in the Avengers world. In a meeting with Percy Jackson and his group, Loki might see an opportunity to cause chaos. He could try to manipulate the situation by pitting the Avengers against Percy and his friends, thinking he can use the confusion to his advantage.