Kagamine LEN is the devil, and Kagamine Rin is the angel. There is a story circulating among them - people from the devil world cannot be together with people from the heaven world.
Finally, it was the group of demons who broke the love. They explained to the demon king's father, and finally took RIN away and burned her alive in the fire.
Thousands of years later, LEN used the crystal ball of the demon fortune teller to arrive in the 21st century, and RIN had already been reincarnated as a human being. Although she is a human being, her dormant power must be unlocked by something.
LEN transferred to their school, preventing many people from killing Kagamine RIN.