It might be a powerful magical being. This being could be on a mission to study the Powerpuff Girls' powers and in the process of capturing Bubbles, realizes she has a very special personality. He is intrigued by her and as they interact more, love blossoms between them.
It could be a new villain who has a soft spot for Bubbles. Maybe he's a misunderstood bad guy who just wants some attention. He might see Bubbles as a symbol of purity and kindness that he lacks in his life, so he kidnaps her in a rather desperate attempt to be close to her.
I'm not sure specifically as 'Bubbles got captive' could have various interpretations in a Powerpuff Girls (PPG) fanfic. It might involve a plot where Bubbles is captured by some villain or in a dangerous situation that the story then unfolds from.
In such a fanfic, perhaps Bubbles would initially be terrified when kidnapped. But as time goes by, she might discover some kindness in her captor. Their relationship could develop gradually. For example, the captor might show her a different side of the world she never knew. And Bubbles, being the sweet and innocent one among the Powerpuff Girls, could start to have feelings for this person who is different from the usual villains she faces. This would create a complex dynamic where she has to balance her new - found feelings with her loyalty to her sisters and the city they protect.
Well, it could also be a fanfic where the demon has some special connection to the Powerpuff Girls' origin. Bubbles might be the one to uncover this mystery. The demon could be a part of an experiment gone wrong in the past that created the Powerpuff Girls, and Bubbles' kindness and curiosity could be the key to resolving the issues related to the demon.
There's a possibility that a new, unknown and powerful monster could be the captor. This monster could be a hybrid of different mythological creatures, created by some dark magic. It has heard of Percy's fame and power and wants to keep him captive either to study him, or to use his power to become even more powerful itself. It could have a lair in a hidden and dangerous place, like an underground cavern filled with deadly traps.
Obviously, Bubbles is one of the main characters. And then there's the demon. There might also be other Powerpuff Girls like Blossom and Buttercup who could play supporting roles, either helping Bubbles deal with the demon or getting involved in the overall situation in some way.
Well, it could be about Bubbles getting interested in a football star Butch. Maybe she goes to his games to cheer for him and they start to become friends. Then they could have adventures together around the football world.
A fanfic could focus on Bubbles making new friends outside of her usual group. Maybe she meets an alien who is lost on Earth and she helps the alien find its way home while learning about different cultures in the process.
It's more of a captor - captive situation. Jabba is a slimeball who holds Leia captive. Leia is in no way in love with him. She endures his presence as a form of resistance and to find a way to escape and defeat him.
The main characters are clearly Bubbles and Butch. Bubbles is one of the Powerpuff Girls, known for her sweet and bubbly nature. Butch is likely a character in the context of being a football star, bringing his own athletic and competitive traits to the story.