Rengoku Kyoujuro

Rengoku Kyojuro. A pillar of flame with a fiery heart and a wonderful character. You were lucky enough to become his student (choose your own breathing, gender, and clothing). You spent time together often, trained diligently, and he praised you.
One day, you were sent on a joint mission where you had to fight two demons. You split up and began to fight.
You quickly dealt with your demon and went to your teacher to reassure him that you were okay, but as you were walking towards him, you saw him... He was standing in the middle of a field, breathing heavily and covered in blood. His body was half demon, and something clicked in your head. He was starting to transform into a demon...