One way is to have it be a key to a mystery. Maybe it has a hidden mark or inscription that leads to a secret location or treasure. Another way is to have it be a tool for a Pokemon's evolution. The trainer uses the hair tie in some special way to help their Pokemon evolve.
The hair tie could be part of a bet or a challenge. For instance, if the trainer loses the hair tie during a battle, they have to perform a certain task. Or it could be used in a magical or supernatural way. In a world where magic exists, the hair tie could have some sort of enchantment on it. Maybe it gives the trainer special powers when they wear it on their wrist, but if it's removed, they lose those powers. This could create a lot of interesting plot twists and conflicts in the fanfiction.
The black hair tie on the wrist can be a part of a character's transformation. Suppose a shy character decides to take off the hair tie from their wrist and tie their hair up, signifying a newfound confidence and a change in their attitude towards a situation in the fanfiction.
It could be a clue. Let's say a character goes missing in the fanfic, and all that's left behind is a hair tie on the wrist. The other characters then have to figure out who the hair tie belongs to and what it might mean in relation to the disappearance. This could lead them on a journey of discovery through the town and its secrets.
One way is to have the character enter the dark tunnel while fiddling with the hair tie on their wrist out of nervousness. Maybe they find something in the tunnel that makes them realize the importance of the hair tie, like it was a key to unlock a hidden door.
In fanfiction, the hair tie on the wrist could symbolize the trainer's connection to a certain Pokemon. For example, if they have a Pokemon that likes to play with the hair tie, it shows a bond between them. It could also be a sign of the trainer's preparedness. They are always ready for any situation, whether it's to keep their hair out of their face or to use the hair tie for some other creative purpose in their adventures.
The character could be in the dark tunnel, feeling lost. They look at the black hair tie on their wrist and remember a promise they made. This memory gives them the motivation to keep going and find a way out of the tunnel. For instance, if it was a promise to a friend who is in danger, the hair tie serves as a reminder of their duty.
It could be used as a way to identify the protagonist. For instance, in a world full of Pokemon trainers, the unique Pokemon hair tie on the wrist makes the main character stand out. Other characters might recognize it and know that the person wearing it is someone special or has a particular connection to a certain Pokemon or event.
One idea could be that the hair tie on Misty's wrist is a memento from a special event. Maybe it was given to her by a friend she made during one of her travels in the Pokemon world. This hair tie could hold some sort of power or just be a symbol of friendship that she cherishes. In the fanfiction, she could be in a tough situation where she looks at the hair tie for strength.
A black hair tie on the wrist can be a link between different storylines or characters. One character might be searching for the owner of the black hair tie they found on their wrist, which leads them to cross paths with other important characters. It could also be a symbol of a character's loyalty. If a character is part of a group, wearing the black hair tie on the wrist shows their allegiance to that group, and any threat to the hair tie can be a threat to the character's loyalty and relationships within the group.
It could be a symbol of a prophecy. In the fanfiction world, there could be an ancient prophecy that mentions a 'band of power on the wrist of a true believer'. Darcy, being an Avengers fan, is the true believer. When the Avengers face a seemingly unbeatable enemy, they start to notice that Darcy's hair tie on her wrist has some strange powers. It glows when danger is near or gives off a force field. This makes the hair tie a central plot device as they figure out how to use its powers to defeat the enemy.
The hair tie could be a key to unlocking a hidden power or a mystery. Say there are ancient prophecies in the Twilight universe that mention a hair tie on the wrist being the key to something big. Writers can build a whole plot around characters trying to figure out what that something is and how to use the hair tie to their advantage.