There's a possibility that some of the other demigods from Camp Half - Blood would step in. For example, Jason, who has his own set of powers and combat skills. He could use his powers of flight and lightning to assist Percy in the search. Also, Piper, with her charm - speak ability, could be useful in getting information out of any captors or witnesses.
Annabeth would most likely be the first one to help Percy. She's his close friend and has always been there for him in difficult situations. Grover could also be a great help. With his ability to sense things, he might be able to pick up on some clues about where Percy's sister is being held.
In this kind of fanfiction, perhaps Percy would go on a heroic quest to rescue his sister. He might gather his friends and allies, like Annabeth. They could face many challenges along the way, such as battling strange creatures or solving difficult puzzles that the villain 'geae' has set up to prevent Percy from reaching his sister.
Chiron, the centaur mentor at Camp Half - Blood, might get involved in the rescue. He has a vast knowledge of the demigod world and could use his wisdom and connections to track down Luke and free Percy. Also, some of the other demigods at the camp who look up to Percy might volunteer to help, like Clarisse. She may not like Percy much, but she wouldn't want a fellow demigod to be in the hands of Luke.
Annabeth could be one of the people to help Percy train. She's very intelligent and could come up with strategic training plans for him. She might teach him how to analyze his opponents' weaknesses more quickly.
Percy would also be determined. He's faced many tough situations before, and this would be no different. He'd draw on his inner strength and the support of his friends. He might feel guilty for not being able to protect his sister in the first place, which would fuel his determination even more to get her back.
It could be Dr. Eggman. He's always after Sonic and would love to get his hands on him to stop Sonic from foiling his evil plans. He might want to study Sonic's speed and use it for his own machines.
The kidnapper could be a former ally who has turned against Luffy. Maybe they've been corrupted by power or some other force. They believe that by kidnapping Luffy, they can take over the Straw Hat Pirates or get their hands on valuable resources that Luffy has access to.
It could be a rival spy agency. They might see Moneypenny as a valuable target to get information about MI6's operations. They would likely be highly trained and well - equipped.
The kidnapper might be a deranged Toad. Maybe this Toad has been corrupted by some evil force and decides to kidnap Luigi for reasons only known to the twisted mind of this new 'evil Toad'.
Maybe it's a character from one of the games Matpat has theorized about. For example, if he made a theory about a game villain being more complex than thought, that villain could come to life in the fanfiction and kidnap him to prove his own power and intelligence.