There is a famous mythology that heavily used in Japanese culture. Japanese called it as Akai ito or Unmei no akai ito which mean red string or red string of fate.
Originated from Chinese legend, this term is widely used in Japanese popular culture especially manga, anime, music and even video games.
So, what is red string/thread?
An invisible red thread connects those who are destined to meet regardless of time, place or circumstance. The thread may stretch or tangle but it will never break.
And there is a story behind this term but before I write about it, I want to stress here that this is just a myth and please take it as another tales and don’t take it too seriously.
A famous story of red thread involves a young boy. Walking home one night, the boy sees an old man standing beneath the moonlight (Yue Xia Lao). He explains to the boy that he is attached to his destined wife by a red thread. Yue Xia Lao shows the boy his destined wife, a young girl. Being young and having no interest in having a wife, the young boy picks up a rock and throws it at the girl, running away. Many years later, when the boy has grown into a young man, his parents arrange a wedding for him. On the night of his wedding, his wife waits for him in their bedroom, with the traditional veil covering her face. Raising it, the man is delighted to find that his wife is one of the great beauties of his village. However, she wears an adornment on her eyebrow. He asks her why she wears it and she responds that when she was a young girl, a boy threw a rock at her that struck her, leaving a scar on her eyebrow. She self consciously wears the adornment to cover it up. The woman is, in fact, the same young girl connected to the man by the red thread shown to him by Yue Xia Lao back in his childhood (source).
In Chinese legend, the gods tie the thread around the ankles of those who are to meet one another but in Japanese culture, the thread is tied around the little finger/pinky.
You can find this term in media such as Inuyasha, Toradora, Bleach, Legend of Zelda, Just Be Friends and Makka no Ito.