Well, if the captor is a child, the fairy might try to trick the child into letting it go. It could tell tall tales about having great treasures hidden in the forest. If the captor is an adult with malicious intent, the fairy would be very defensive and try to hide its magic from being exploited, while constantly looking for a chance to break free.
In this fanfiction, the captured fairy in the jar could be part of a scientific experiment in a magical world. Scientists are trying to understand the nature of fairies. The fairy is scared and tries to communicate with the outside world. A kind - hearted assistant at the lab might start to feel sorry for the fairy and plot to help it escape, leading to a series of adventures and moral dilemmas.
It could be Captain Hook. He has always been Peter Pan's nemesis and has a strong motive to capture him. Maybe he wants to finally get revenge for all the times Peter has outsmarted him.
There's a possibility that it's a villainous trainer. This trainer might capture Gengar with the intention of using it for evil purposes, like using Gengar's powers to steal other Pokémon or cause chaos in Pokémon battles.
It could be Discord. He loves causing chaos and might capture Twilight Sparkle just for the fun of it or to disrupt the harmony in Equestria.
Maybe it's an old foe who has managed to regain power and wants to make Sora pay for past defeats. For example, a powerful dark sorcerer who was once defeated by Sora but has now found a way to capture him through some devious plot.
It could be Team Rocket. They've always been after Ash and his Pikachu. They might finally succeed in capturing him and then Pikachu has to come up with a plan to rescue Ash.
Another possibility is a group of corrupted Plumbers. They turn against Ben and capture him, perhaps because they want to use the Omnitrix for their own evil purposes. They would have knowledge of Ben's abilities and how to contain him, which would make for an interesting conflict.
The captor might be initially cold and distant, seeing the mermaid only as a curiosity. But as time goes on, they could become more empathetic. For example, if it's a fisherman who captured the mermaid, he might start to feel guilty when he sees her sad eyes. The mermaid, on the other hand, would be hostile at first, trying to bite or splash the captor. But she might also try to communicate her needs, like asking for fresh water or more space in the tank.
Maybe some mad scientist who wants to experiment on Beast Boy's shape - shifting DNA. He could be looking for a way to transfer the ability to himself or create some sort of mutant army. Another option is that it's a group of aliens who mistake Beast Boy for a dangerous creature and capture him to study. They might not understand that he's a sentient being with friends who will come looking for him.
Bardock would probably be initially cautious around the Fairy Tail characters. Given his Saiyan nature, he is used to a more cut - throat and survival - based environment. But as he gets to know them, he would start to respect their loyalty and sense of family. For example, he might be impressed by how the guild members always have each other's backs, much like how Saiyans rely on their comrades in battle.