He would probably be very surprised. Percy might start by trying to convince his younger self that he is indeed from the future. He could also be a bit embarrassed by some of the things he did when he was younger.
In this fanfiction, Percy might be shocked at first to see his younger self. He could offer advice to his younger version, like how to better handle the challenges in the magical world. Maybe he'd warn his younger self about certain enemies or mistakes he made in the past.
Percy would likely be very critical of their lifestyle. He comes from a world where family is important and they support each other in the face of danger. The Dursleys, on the other hand, seem to shun anything out of the ordinary and are unkind to Harry. Percy might try to show them a different way of living, with more love and acceptance, though they probably wouldn't listen.
At first, Percy would likely be angry. He's a fighter, and being betrayed by someone he loves would fuel his rage. He might go after Annabeth to demand an explanation. But as the reality sinks in, he could become very sad and withdrawn. He might isolate himself from his other friends as well, feeling like he can't trust anyone anymore. He would constantly be thinking about what he did wrong to make Annabeth turn on him.
He would probably be shocked. Percy has always trusted Annabeth, so being stabbed by her would be completely unexpected. He might initially be unable to believe what's happening.
Percy would probably be angry at first. He has a strong sense of justice and being kidnapped against his will would make him fight back. He might immediately try to use his water powers to break free or at least create a distraction.
At first, Naruto could be a bit skeptical. He might think it's some kind of trick or illusion. But once he realizes it's really his future self, he would be excited. He would want to know about his future family, if he has one. He'd also be interested in the state of the ninja world in the future. His reaction would be a mix of excitement, curiosity, and a determination to make sure his future turns out as good as his future self's seems to be.
Thor would be the most likely to relate to her on some level, as he comes from a world of gods and magic too. He might see her as an equal in a way. Bruce Banner would be interested in the scientific aspect of how her powers work, if there is any scientific explanation within the context of the fanfiction. And the others like Clint Barton and Wanda Maximoff would be curious about her background and how she ended up in their world.
Percy's character could become more self - aware. Seeing his past self would make him reflect on how far he's come. He might realize that he's grown more confident and capable over time.
Percy would likely be very confused at first. He'd probably feel a mix of anger and sadness. He might start to question his own actions and wonder if he did something wrong to make his sister leave.
Percy would feel a deep sense of guilt as well. He might think that he should have been there to protect her. So, he would be determined not only to free her but also to make sure whoever is responsible pays for what they've done. He would use his powers over water to his advantage, like creating distractions or using water to weaken the enemy's defenses while he tries to reach Annabeth.