The fact that her hair is in a bun might also play a role. It could be a cultural or traditional style in the fictional world she belongs to. When she's kidnapped, her captors might be disrespectful of this style, which angers her and gives her the motivation to resist. The bun could also be used practically during her captivity. For example, she might hide small objects in it to help her escape, like a lock pick or a small note.
Such a description might be used to make the character stand out in a fanfiction. He could be a unique character in a fantasy - based fanfic too. For example, he could be a wizard who has adapted to some modern - day human habits like wearing a hair tie on his wrist. His bun could be a way to keep his long hair out of his face while he's performing magic.
In hair fanfiction, a black hair tie on the wrist can symbolize a connection to a particular character. It might be something that the character left behind or a memento. For example, if it was from a love interest, it could represent an unspoken bond or a longing for that person.
Perhaps in the fanfiction, the black hair tie on Jade's wrist is a clue to her past. She has amnesia and the hair tie is the only thing she has from her previous life. As she begins to investigate, she meets various characters who either try to help or hinder her in uncovering the secrets associated with the hair tie.
Perhaps the black hair tie is a reminder of a past love. Felicity is constantly haunted by the memories associated with it. She meets someone new in the fanfiction, and the presence of the hair tie creates a conflict as she tries to move on from her past while still holding onto this memento.
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.
In the fanfiction, her hair tie could be enchanted. When she's kidnapped, she discovers that she can use it to send out a distress signal only visible to those she trusts. Her bun could also hold a secret. There could be a hidden tattoo on the back of her neck that is only visible when her hair is in a bun, and this tattoo is the key to a great treasure. The kidnappers don't know this yet, but they will soon find out as the story unfolds.
There are many fanfiction platforms out there, but it might be a bit difficult to specifically find ones with this exact description. You could start by looking at platforms like Archive of Our Own (AO3). Try searching for tags like 'Male with hair tie' and 'Hair in bun' along with the fandom you're interested in. For example, if it's a Harry Potter fanfic, add 'Harry Potter' to the search terms.
A fanfiction could revolve around a mom with a black hair tie on her wrist who is a superhero in disguise. The hair tie is actually a magical object that gives her powers when she ties her hair back. She has to keep it a secret from her family while still saving the day in the neighborhood.
In fanfiction, a black hair tie on the wrist could symbolize a connection to a particular character. Maybe it was given by a love interest, and the character wears it as a reminder of that person. It could also be a sign of a secret society or group within the fanfiction world, where the black hair tie is a form of identification.