The Salvatores, Damon and Stefan, are likely candidates. They have always been protective of Elena and would do anything to save her. They could use their vampire skills and knowledge of Kol's weaknesses to stage a rescue.
Caroline might play a role in saving Elena too. She's strong, both physically as a vampire and also in terms of her determination. She could team up with the others, like Tyler if he's in the picture, and they could all work together to rescue Elena from Kol. They could come up with a clever plan that involves using Kol's overconfidence against him.
Bonnie could also be the one to save Elena. As a powerful witch, she could use her magic to track Kol and Elena down. She might even be able to conjure up some spells to defeat Kol and free Elena. And if Jeremy is still around, he could also help out, given his skills as a hunter.
One possible plot could be that Kol kidnaps Elena to use her as bait for his enemies. He might think that by having Elena in his custody, her friends, especially the Salvatores, would come to rescue her and he could then ambush them. Another idea is that Kol has some sort of vendetta against Elena because of her connection to the doppelganger bloodline. So he kidnaps her to extract revenge on the entire line.
They could develop a shared hobby. For instance, they both get into painting and use it as a way to express their emotions and connect on a deeper level. It would be unique because it shows a side of them that's not related to their supernatural natures.
I'm not sure specifically as it depends on the particular fanfiction. It could be a story where a character named Kol kidnaps Bonnie within the universe of the show or book that the fanfiction is based on. Maybe it's part of a complex plot involving magic, power struggles, or personal vendettas.
Elena would likely be angry. She has her own agency and doesn't like being controlled. If Stefan kidnapped her, she would fight back. She might use her knowledge of the supernatural world and her relationships with other characters to try to escape. For example, she could try to call out to Damon telepathically if they have some sort of connection in the story.
She could be terrified at first. After all, Klaus is a very powerful and dangerous vampire. But Elena is also a strong - willed character. So she might quickly gather her courage and start looking for ways to escape.
The Salvatore brothers, Stefan and Damon, are the most likely candidates. They both have a strong connection to Elena and would stop at nothing to rescue her. Stefan's sense of chivalry and Damon's fierce protectiveness would drive them to track down Katherine and get Elena back.
The main characters are clearly Kol and Bonnie. Kol is the one doing the kidnapping, and Bonnie is the victim of this action.
One common theme is forbidden love. Since Kol and Elena might be from different 'worlds' or groups within the show's lore, their relationship can be seen as something that goes against the norm. Another theme could be passion. The kiss is often a moment of intense passion that drives the story forward.
The plot might involve Kol falling in love with Elena. Kol, being the wildcard among the Originals, could try to win Elena over in his own unique way, which might create a lot of drama with the other characters like Damon and Stefan who are also interested in Elena.
Another character that could be important is Elijah. He is often the voice of reason among the Originals. He might try to guide Kol in his relationship with Elena, or he could be against it if he thinks it will bring trouble to their family. So in this fanfiction, he could be a key figure in the development of the Kol - Elena relationship.