Integrity

Integrity is a soul that fell into the Underground and then got captured by Asgore. They are shown to possibly have been a ballerina, as their weapon found are some ballet shoes and an old tutu as armor. Integrity is also the soul used in Sans (Genocide)'s battle, where Sans can transform Frisk's soul into blue, which is shown to not be able to fully move around the Bullet Board. And, as seen by Sans, it can also be commanded to move to another side of the Board (with violence, so it will make HP lower).

Trivia

 * Integrity was captured by Asgore, and they were surely a ballerina, as seen by their items that can be found.