Why is Tsunade afraid of blood?

As a young adult, however, she developed haemophobia, which is the fear of blood. The phobia developed following the death of the man she thought she would spend the rest of her life with. ... When he died, her hands were covered in his blood, and the sight haunted her for a long time.

What is the mark on Tsunade's head?

The Yin Seal: Release is a powerful sealing technique used by Tsunade. Over a period of time, Tsunade will focus and build up chakra in the seal mark on her forehead. The release command will disperse the seal and allow the stored chakra to pump back into her body.