Сила

Strength is Yuu's darken persona in the other world. She wears long black boots similar to Black Rock Shooter's, black spandex and a white flamed embroidered hooded scarf. She has a vertebra like tail with white hair. Her main weapons are her two large black "Ogre Fists" that she wears over her arms and are as big as she is. In the TV series, she gained emotions when she established a connection with Yuu just as she was about to die following a fight with Black Rock Shooter. When Yuu preferred to live in her world than reality, Strength was forced to switch bodies with Yuu, although she doesn't cast a shadow.

She inserted false memories of herself into Mato to make it seem like they've been friends for a long time, though she eventually came to like Mato herself. She has the ability to put herself or others into a comatose state, sending their mind into the body of their darken persona. At the series conclusion, she disappears into the wind, returning Yuu to her original body, although she revives later on to continue fighting. (Source: Wikipedia)