She Has Guarded Me for Seven Lifetimes, but She Is Not My Biological Sister

In seven reincarnations, she has guarded me for seven lifetimes. I always thought she was my biological sister. Until the master said before he died: "Believe her, don't let her be alone." Every day she would take out the blue thread and wrap it around the button several times. Sixty-seven laps, never less. I asked her what to count. She didn't say. Later I learned that what she was counting was not a line, but a countdown to her life. My name is Lex, a hybrid of wolf and vampire, and a "forbidden son" who is hunted by two races. The two blood vessels in my body clashed, and everyone said I was a waste. There was only her, walking barefoot in the snow, following me from the northern region to the south. She said that the meaning of her life was to protect me. But she is not my biological sister. So who is she? The secret of the seventh life is hidden in that blue thread. In this life, it's my turn.