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.
In fanfiction, the hair tie on the wrist could act as a tracker. A group of villains might be after a special Pokemon that has a connection to the person wearing the hair tie. The villains can sense the presence of the hair tie and try to follow the protagonist to get to the Pokemon. This creates a lot of tension and action as the protagonist has to constantly outwit the villains while protecting the Pokemon and the secret of the hair tie.
The Pokemon hair tie on the wrist can be a key to unlocking a hidden power or location. Maybe there are ancient ruins that only respond to the presence of the hair tie. The protagonist has to figure out how to use it in the right way to access these secrets. This could involve solving puzzles or meeting certain Pokemon that are related to the origin of the hair tie.
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.
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.
The hair tie on the wrist could be a tracker. In a fanfiction where dark forces are after a character, the hair tie could be enchanted in such a way that the bad guys can sense its location. This would create a lot of tension as the character has to either find a way to hide the signal or use it to their advantage, like leading the villains into a trap. It could also be a way for the good guys to find the character if they get lost or kidnapped. For example, a group of Hogwarts students could use a spell to track the hair tie's magic signature. Another way it could be a plot device is if it has the power to transfer memories. When a character touches it, they might start seeing visions of past events related to the Harry Potter world, which could lead them on a quest to uncover the truth about something that happened long ago. This could be a crucial part of the story's mystery and drive the plot forward as they try to piece together the fragmented memories.
The hair tie on the wrist could be a key part of a mystery. Suppose a character finds a strange hair tie on their wrist and doesn't remember how it got there. This could lead them on a journey to figure out what happened during a time they can't recall. It could be related to a blackout or some sort of mind - altering event that is common in the Pretty Little Liars universe.
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.
It can be a clue. For instance, the hero in the fanfiction might notice the hair tie on the wrist of a suspect and start to suspect that person of being involved in the kidnapping. It's a simple but effective way to introduce an element of mystery and investigation into the story.
It could be used as a way to identify Bella in a dangerous situation. If she is kidnapped or in trouble, the hair tie could be the one thing that gives her away or helps her friends find her. For example, if she leaves a piece of the hair tie behind, her vampire or werewolf friends can track her scent using it.
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.
It could be used as a way to trigger memories. For example, if the hair tie was from a particular person, whenever Lorelai looks at it, she remembers something important related to that person, which then drives the plot forward.
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.