She is a co-operative, easygoing character and well suited to be Black Star's partner in the show Soul Eater.
She is Black Star’s weapon. She can change into multiple ninja weapons like shurikens, smoke bombs, and a powerful katana later in the series. She has a very forgiving and easygoing personality and this is why she is able to work well with Black Star. Most people wouldn't be able to put up with Black Star's loud, boisterous and egotistic personality but it's something that Tsubaki finds endearing in him; Tsubaki's soul is “very co-operative and has a lot of space to accept people” as said by Dr. Stein.
Tsubaki grew up in Japan and her family was very high ranking in nobility and standing. She was always very quiet and went along with whatever her brother Masamune wanted. Her name, Tsubaki, means a flower that has no scent, and she reflected that by never really speaking out or standing up for what she wanted, she just obediently went along with whatever those around her wanted.
When she arrived at Shibusen to study to be a Weapon, she saw Black Star standing on top of the school proclaiming to everyone how great he was and how he would eventually be greater than god. She was the only person who stayed throughout the entire speech and applauded him at the end. It was then that he decided to team up with her and become her meister. However this turned out to cause many problems for her, as she had to teach him from the ground up how to be an assassin. He could never get down the habit of being quiet and sneaking up on the enemy. It was because of this that despite Tsubaki's hard work, they never captured one soul. They attempted to attack and kill Mifune who possessed enough strength to be equal to 99 souls. However when they found out he was protecting a small child, Angela, they left him alone and continued their journey to gain enough souls for Tsubaki to be a Death Scythe.