What's with the long hair in the back?

If it is a small amount, it is normal, and the human body will grow some hair when it reaches puberty.

If the quantity is large, the following conditions should be considered:

Food source: excessive hormone intake.

Genetic inheritance: there may be some inheritance in parents' hair.

Supplement: If you feel that it affects your appearance, you can go to a professional hospital for hair removal. Don't buy depilatory cream yourself, it may cause allergies and lead to some side effects.