She is Black Star's Dark Demon Blade partner. She can change into multiple ninja weapons (shuriken, smoke bomb, a katana [later chapters], etc). She is most of the time disheartened by Black Star's loud self but she still puts up with it because that’s what she likes about him. She has a very forgiving personality and always tries to help everyone even when she has no idea how to help. Tsubaki soul is "very co-operative and has a lot of space to accept people" as said by Dr. Stein.