Maybe her best friend, who is really brave and loyal. They have a deep bond, and the friend won't rest until Rose is safe. Her friend could be really smart and figure out the master's hiding place through some clever detective work.
In the 'rose gets kidnapped by the master fanfiction', it might be that the master kidnapped Rose to force her into doing something against her will. For example, he could be a villain in a magical world and wants Rose to use her special powers for his own dark purposes. Her kidnapper could be holding her in a hidden lair, which is full of strange and dangerous things. There might be a long and drawn - out struggle between Rose trying to free herself and the master trying to keep her under his control. This could involve a lot of psychological warfare as well as physical confrontations, depending on the nature of the fanfiction.
Often, this sets off a chain of events that involve complex time-travel shenanigans and intense confrontations. The Master might have some devious plan that the Doctor has to figure out to save Rose.
There could be a mysterious wizard. This wizard might be the one who sets the miracle in motion for Rose. He could be an old and wise figure, guiding Rose through the strange new experiences that come with the miracle.
In certain fanfictions, a jealous rival of Amy Rose could be the kidnapper. This could be another female character who has feelings for Sonic and sees Amy Rose as a threat. She might think that by getting rid of Amy Rose, she can have Sonic all to herself. Or it could be a character from Amy Rose's past who has some unresolved issues with her and decides to kidnap her to get revenge or make her pay for something that happened a long time ago.
It could be an enemy from one of his past cases. Someone who holds a grudge against L for getting them caught or ruining their plans before.
It could be a rival guild member who wants to use Juvia as leverage against Fairy Tail. Maybe someone jealous of her powers or her relationship with Gray.
The Death Eaters are an obvious choice. They've always been after Harry, so they could easily be the ones to kidnap him. They'd want to take him to Voldemort to prove their loyalty and also to finally get rid of Harry for good.
In some 'damian gets kidnapped fanfiction', the kidnapper could be an otherworldly being. Let's say an alien or a magical creature that has an interest in Damian for some strange reason. It could be that Damian has a unique energy or power that the otherworldly being wants to study or harness. Or perhaps it's a human who has been corrupted by some dark magic or technology and is acting on behalf of a more powerful, hidden force that wants Damian for its own nefarious purposes.
Maybe it's a group of poachers who are after rare Pokémon. Gengar, being a relatively unique and powerful ghost - type, would be a valuable catch for them. They could plan to sell Gengar on the black market for a high price.
His friends like Tails and Knuckles are likely to try and save Sonic. Tails is smart and could come up with a clever plan, while Knuckles has the strength to fight off Scourge if needed.