That wolf tooth is a ronin named 'Kui Yi'.

When Yue Ya'er's father saw him, he was already a werewolf, he was adopted by Yue Ya'er's grandfather as a righteous son, and he has been hiding his identity for him, and he took on the responsibility of protecting Yue Ya'er's father in order to repay their family.

Yue Ya'er's grandfather is a very powerful demon hunter, which also attracts many people to hold grudges with demons, so it is very common for Yue Ya'er's father to get lost or even injured, until that day, Yue Ya'er's grandfather brought Kui Yi home.

He was wrapped in a cloak by Yue Ya'er's grandfather, his eyes flashed with green light in the black fluff, he was very scared, he looked at everything with hatred, but he obeyed Yue Ya'er's grandfather, Yue Ya'er's grandfather pointed to Yue Ya'er's father, who was only 6 years old at the time, and said to Kui Yi that he was a younger brother.

Kui Yi looked at Yue Ya'er's father, the light in his eyes gradually stopped cold, and his mouth repeated the word 'brother' repeatedly.

Since then, Kui Yi has been following Yue Ya'er's father, going to school, skipping class, going out to play, and going home. Brother, Kui Yi didn't know where to hide, he suddenly jumped out and pounced on one of the students, grinning, revealing sharp teeth, sticky saliva kept flowing out of between his teeth, dripping on the student's face, he was wearing a black cloak, other students couldn't see his appearance clearly, only the student who was pressed under him saw clearly that it was a wolf's face, and instantly peed his pants in fright, and fainted after a while, the shouts of the students attracted the teacher, Yue Ya'er's father patted Kui Yi, he jumped up from the student's body and ran back, no one knew where he hid。 Since then, no one in the school has dared to bully Yue Ya'er's father, because they know that Yue Ya'er's father has a very scary brother.

Werewolves have a very short lifespan, in the 16th year of Kui Yi's arrival at Yue's house, Kui Yi died, and when he died, he pulled out his tooth and stuffed it into Yue Ya'er's hand, which was less than a year old at that time, and since then, the wolf tooth has been hanging on Yue Ya'er's neck.

Yue Ya'er's grandfather said that Kui Yi was saved by him from another demon hunter, and he was curled up in the corner at that time, covered in scars from being whipped by the demon hunter, but even then, he didn't get up to attack the demon hunter, and Yue Ya'er's grandfather couldn't bear to buy him. Later, Yue Ya'er's grandfather asked him why the demon hunter beat him like that, and he didn't fight back, Kui Yi said that the demon hunter saved him when he was still a human, and he came back this time to repay his kindness, but the demon hunter didn't recognize him anymore.