Goku would be more likely. He is always open to new friendships and challenges. He has a curious nature and would be intrigued by Kefla's power. So he might approach her first to test her strength or just out of friendly curiosity.
It could be Kefla. If she has heard about Goku's great power and his reputation as a fighter, she might be the one to initiate contact. She could see him as a worthy opponent or someone from whom she can learn a lot, which could lead to a friendship.
In some cases, it could be a mutual thing. Maybe they are thrown into a situation where they have no choice but to interact. They both sense the power in each other and that mutual respect could lead to an immediate, unspoken understanding that forms the basis of a friendship.
It could be Louis. He has always seemed very connected to the group's roots. In fanfiction, he might realize how much the fans miss the band and how much they all still have in common musically. So, he starts reaching out to the other members one by one to see if a reunion is possible.
It could be Naruto. He's known for his boldness and his ability to reach out to people. He might be the one to first notice Korra's unique qualities and be drawn to her, starting conversations and gradually building a connection that could turn into a romantic one.
Tim Drake from the Batfamily might be likely. He's highly intelligent and sometimes over - thinks situations. If he discovers some unusual activity related to Young Justice that he misinterprets, he could be the one to start the ball rolling on a conflict. For example, if he sees some new Young Justice members using untested tech that he thinks could be dangerous, he might take preemptive action against them.
However, Luke could also be the one to start the conversation. He is a young and idealistic Jedi, full of hope. He might want to reach out to his father, believing that there is still good in him. He could start the conversation with an attempt to understand Vader's journey to the dark side.
It could be Captain America. He is often the one to reach out and try to make peace or form alliances. He would see the potential in the Maximoff twins and approach them with an open mind and a desire to work together for the greater good.
I think it might be Nico. Given his experiences with different types of beings in his own world, he might sense the supernatural nature of the Winchesters and approach them first, either out of curiosity or because he thinks they could be allies in his ongoing battles against the forces of the underworld.
Koro - sensei could be a likely candidate. Since he has advanced abilities, he might be able to figure out time travel in some way. He could use it to try to change his own past or to better prepare the students for their future.
Itachi. He's one of the more complex and morally gray members of the Akatsuki. If he saw that the female OC was in a situation that could potentially cause chaos or harm to the ninja world in a larger sense, he might initiate the rescue. He's also the type who might see some value in the OC that others don't.
It could be someone like Frieza. Frieza is extremely powerful and evil in the Dragon Ball universe. If he somehow found his way into the Naruto universe or if the fusion character was in the Dragon Ball universe, he would be a great threat. His ability to transform and his cold - blooded nature would make him a tough opponent for the fusion of Goku and Naruto.
Leonardo would be a likely candidate. He is often the most level - headed of the turtles and believes in honor and justice. He might see the potential for a greater good if he can end the hostilities with Tigerclaw, so he could be the one to make the first move towards a truce.