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.
Killua is likely to be completely devastated. He might go into a state of shock at first, not believing that Gon is really gone. Then he could become very angry, either at himself for not being able to protect Gon or at the situation that led to Gon's death.
Gon in fanfiction can be very single - minded in his pursuit of goals, just like in the original. He might be a bit more naive at times, which can lead to interesting plot twists. Killua is often portrayed as having a complex emotional side. He struggles with his past as an assassin and his loyalty to Gon. His cool exterior may crack in some fanfictions, showing his true feelings.
There are also those who are cautious. They may be aware of the potential power imbalance and be hesitant to get involved. They might worry about being used or losing their independence.
Darth Vader would likely be conflicted. On one hand, he could see the potential in his younger self to become a powerful Sith. On the other hand, he might be reminded of his own lost innocence. He might give his younger self some warnings about the path he is about to take, but also encourage the dark journey.
Toge Inumaki is important too. His special cursed speech ability makes him stand out. Also, the principal of the school, Masamichi Yaga, is likely to be important as he is in a position of authority and has an influence on Gojo and the other students. There could be other characters like the members of different clans that also play roles in the complex web of the story.
He might start with basic physical training. You know, like running up mountains or doing a lot of push - ups to build strength. Also, chakra control exercises would be key. He could teach his younger self to focus chakra on different parts of the body for more effective jutsu use.
Well, it could have a huge impact. Naruto, being more experienced, can teach his younger self new jutsus. He might also instill in him a stronger sense of self - belief. For example, he could show his younger self how to better control the chakra of the Nine - Tails. This would make his younger self more confident in battle and in dealing with the village's expectations.