It could also be a new character that is introduced in the fanfiction. Maybe a young girl who has her own set of powers related to light. She discovers Jamie's kidnapping and decides to help. She uses her light powers to dispel the darkness around the place where Jamie is held, blinding the kidnappers and allowing her to free Jamie and lead him to safety, and later on, she might even become an ally to the Guardians.
In this fanfiction scenario, Jamie could be kidnapped by a group of nightmare creatures who have broken away from Pitch's control. They might believe that Jamie holds the key to a new world order where nightmares rule. The Guardians would have to work together more closely than ever. North would use his workshop and his knowledge of the world to strategize. Bunny could use his tunnels to get to places quickly, and Sandy would send out his dream sand to try and communicate with Jamie in his sleep, all while trying to figure out how to defeat these rogue nightmare creatures and rescue Jamie.
Yoshi could also be the rescuer. Yoshi has his long tongue that can be used to grab things from a distance. He could use it to pull Luigi out of a locked cage or something. And Yoshi is very fast and can jump high too. So he'd be a great candidate to go on a rescue mission for Luigi.
Kagami could also be a likely rescuer. He and Kuroko have a special partnership on the basketball court. Kagami would be furious if someone kidnapped Kuroko and would use his strength and determination to find and rescue him. He might track down the kidnappers through his street - smart skills and his knowledge of the areas where Kuroko could have been taken.
Perhaps an escaped convict who has a grudge against the Reagan family in general and sees Jamie as an easy target. This convict might be looking to use Jamie as a bargaining chip to get out of the country or something similar.
There's a chance that Jamie himself could rescue himself. He might be really smart and find a way to outwit Phil Sanfino. Maybe he discovers a weakness in Phil's security or manages to turn Phil's minions against him. This would make Jamie a very strong and independent character in the fanfiction.
A kitchen maid could be the most unexpected rescuer. She might overhear the kidnapper's plans while serving food in the castle. She's not someone you'd expect to be heroic, but she could use her knowledge of the castle's nooks and crannies to outwit the kidnapper and save the daughter.
A rival within the police force or someone who wants to discredit the Reagans could also be the kidnapper. They might think that by kidnapping Jamie, they can make the family look bad and gain an advantage in some internal police politics. Maybe they are jealous of the Reagans' reputation and want to bring them down a notch.
In this fanfiction, it might be that Phil Sanfino is under the control of some other force. He kidnaps Jamie as part of a larger plan. Maybe there's a mysterious organization pulling the strings, and they need Jamie for some experiment or ransom. The story could then revolve around Jamie trying to escape and figure out what's really going on while also dealing with Phil, who may or may not be a willing participant in the kidnapping.
Pitch Black would be a likely main antagonist. He could manipulate Jack Frost into turning dark, as he has a history of preying on the insecurities of the Guardians.
Most likely Naruto would be one of the main characters. Since it's a Naruto - based fanfiction, he'd probably be at the center of the story. And maybe some of the Guardians from 'Rise of the Guardians' would also be main characters, like Jack Frost.