Originally an art drawing title "The Strength." She would go on to appear in the manga Black★Rock-chan, and in different capacities in Black★Rock Shooter OVA and Black★Rock Shooter (2012)
Su-san (Strength) is a side character and comedic foil for Black★Rock-chan. She doesn't seem to have a home or family. According to De-chan she just wanders wherever food is, as she seems to be perpetually hungry. Toward the end of the series she takes up residence with Golden Sensei. Her ogre arms appear and disappear depending what's happening at the moment. Her personality tends to be stoic and she talks even less than Black★Rock-chan
Black★Rock Shooter OVA
Strength is the other self of Yuu Koutari, a somewhat forgettable side character. She's really more of a plot device. Strength has no speaking lines and only appears a few times cryptically overlooking a cliff edge. Yuu Koutari acts as the basketball club coach, which is how she initially meets Mato, the main star of the OVA. During the second school year she shares some classes with Mato, which contributes to Yomi becoming jealous and the central plot.
Black★Rock Shooter 2012
Strength is the other self of Yuu Koutari, a friendly but quiet girl who is has been best friend's with Mato since kindergarten. Unlike in the OVA, here Yuu comes inadvertently between Mato and Yomi, which leads to the division between Mato and Yomi, and leads into the main plot line. Later it is revealed that she has some connection to Saya when she accuses her of changing Yomi. Towards the end Mato becomes confused when mysteriously no one seems to remember or even know who Yuu is. Yuu sends Mato to the other world to try and stop Black★Rock Shooter's kill spree, only for Mato to to join just as Black★Rock Shooter kills Dead Master, the combined emotional pain and horror causing Black★Rock Shooter to become Insane. Strength battles with Insane Black★Rock Shooter, saving Black★Gold Saw from being killed and taking Black★Gold Saw to her (Strength's) realm, but Insane Black★Rock Shooter wedges two swords into the portal and follows after them.
During the fight Yuu joins with Strength despite Saya's orders. It is revealed that Yuu and Strength traded souls at some point in the past, so "Yuu" was actually Strength and "Strength" was actually Yuu. Away from the fight Yuu (Strength) tries to kill herself to send Strength (Yuu) back to the human world to save her from Insane Black★Rock Shooter. Strength along with the other selves gives her power to Black★Rock Shooter (Mato) to create the Super Rock★Cannon. After the fight she is shown to be dying from her injuries and says her final goodbye to Mato and Yuu, comforting her and encouraging her to resume life in the human world. In the post credits scene Strength is shown to have been fully resurrected and ready to fight until she see's Black★Rock Shooter is different now.