Other self

An other self (思念体, shinentai) is an alternate counterpart to a human character that resides in the Hollow World. They are generally described as solitary beings who have the ability to defend themselves using their own weapons, seemingly reflecting the portrayal of the Hollow World as a place filled with dangers. As alternate counterparts, they possess an invisible connection with their respective human counterparts in the real world. The concept of other selves are prominently featured in the Black★Rock Shooter OVA, where they were first introduced, and Black★Rock Shooter 2012 anime.

Black★Rock Shooter (OVA)
Although the Black Rock Shooter OVA was the first to introduce the concept of other selves, little is actually explained of who they are or what purpose they serve exactly, and can only be inferred from what is shown. The OVA appears to define an "other self" as a typically silent being that has some sort of connection or personality similar to that of their human counterpart. The visual cues that an other self is one to a human counterpart are similar eye colors and hairstyles.

When they reach a common goal, the other selves can create a connection to their human counterparts or vice-versa and make a pact with them to "merge" or "fuse". Mato Kuroi and her other self, Black★Rock Shooter, were shown doing this at the end of the OVA, and it can be assumed that Yomi Takanashi and Dead Master did the same as well. While merged with each other, how much control each party has is also unknown. When merged with Yomi, Dead Master was behaving in a cruel and violent manner, much unlike Yomi (though it is possible Yomi had lost control or was sharing Dead Master's angry instincts). However, Black★Rock Shooter seemed to be empathetically reacting to Dead Master/Yomi in a way Mato would be likely to.

It is possible that an other self cannot even materialize without merging with their human counterparts, as Black★Rock Shooter's lips do not move when she talks before she merges with Mato, and Dead Master dissolves immediately upon leaving Yomi's body.

Only Black★Rock Shooter and Dead Master were explicitly confirmed to be the other selves of their respective human counterparts, Mato and Yomi. The visual cues listed above imply that Strength is the other self of Yuu, as both have the same choppy hairstyle and hazel eyes, though nothing was shown of their connection to each other. While Black Gold Saw resides in the Otherworld, whether she is an other self at all is unknown.

Black★Rock Shooter (TV Anime)
The concept of other selves in the Black★Rock Shooter TV Anime series is similar to that of the OVA's, carrying over the idea of other selves as "otherworldly beings that possess a connection with a human being". However, the anime portrays the connection between the two in a more evident way and expands upon the concept.

Other selves are alternate versions of their human counterparts, born of the Otherworld. They spend their entire lives fighting while bearing their human counterparts' darkest thoughts and fears for them until the bitter end, where upon an other self's death, their human counterpart forgets their pain and what was causing them to suffer, along with the feelings and memories associated with their pain. Other selves do not have emotions of their own, and their actions reflect the feelings of their human counterparts. Strength was no exception until she connected with Yuu Koutari for the first time, allowing her to feel emotions and sense how badly Yuu was suffering in the real world. Strength would come to understand why the other selves fight for their human counterparts, but also sympathized with how the other selves were suffering themselves in the face of constant battle, and agreed with Mato Kuroi disapproving of someone else "having to die for [her] to solve all [her] problems", saying that differences should be resolved directly by talking.

Saya Irino implied the existence of Mato's other self to her by explaining that there is someone out there who takes a person's pain from their hearts so that they will never feel physical pain when they are hurt. That someone is Black★Rock Shooter, who is particularly notable because she reflects Mato's self-sacrificing nature to an extreme, stopping at nothing to destroy other peoples' worst feelings, carried by other selves, without mercy. This resulted in her being as powerful as she is. As stated above, the death of an other self causes their human counterpart to forget their pain and the feelings and memories associated with it. This has been shown three times in the anime: the first was Kagari Izuriha forgetting her past feelings towards Yomi after Chariot's death; the second was Arata Kohata forgetting her memories of Taku Katsuchi after her other self was beheaded; and the third was Yomi Takanashi forgetting all about Mato after Dead Master was killed. All three of their other selves' deaths were at the hands of Black★Rock Shooter.

Since the other selves' actions reflect the feelings of their human counterparts, the battles between other selves also often reflect the experiences of their human counterparts in real time, as shown when Black★Rock Shooter fights Chariot at the same time that Mato and Kagari conflict with each other with Yomi at the center of their conflict. The outcome of these battles, as with the strength of the other selves involved, would also reflect the resolution of that conflict in the real world. At times, the real world can sometimes influence the Otherworld; the voices of human counterparts can be heard in the Otherworld, while a human's physical status may reflect in the Hollow World as well (for example, Chariot's wheeled legs reflecting Kagari's use of the wheelchair and the heart-shaped scar on Yomi's and Dead Master's chests).

It is later shown that human counterparts can enter the Hollow World by transferring their consciousnesses into the bodies of their respective other selves. How this is done exactly is unknown, but Yuu and Saya are capable of doing so, with Yuu even being able to transfer Mato's consciousness into Black★Rock Shooter. Under normal conditions, the human counterpart can return to the real world by waking up, but Mato became trapped within Black★Rock Shooter after her trauma caused the latter to transform into Insane Black★Rock Shooter. She was able to separate herself from Black★Rock Shooter (but still remain in the Otherworld) and Strength from Yuu by pleading to protect Yuu from Insane Black★Rock Shooter.

Despite the fact that other selves do not have emotions of their own, they have their own free will, and rather than being forced to, they choose to fight out of love for their human counterparts. Strength says that this is the reason the other selves could never fight for someone they hate, because regardless of their individual personalities, all the other selves have a deep and unconditional love for their human counterparts, whom they regard as the most precious person to each of them.

All the known other selves are distinguished by their shared characteristics: they have a concentric circle pattern in their eyes, a signature color theme, and blood that is both black and their respective signature color (for example, Black★Rock Shooter's blood is black and blue). They also each have a realm that belongs to them within the Hollow World.

Black★Rock Shooter (OVA)

 * Black★Rock Shooter
 * Dead Master
 * Strength (implied)

Black★Rock Shooter (TV Anime)

 * Black★Rock Shooter
 * Dead Master
 * Strength
 * Chariot
 * Black Gold Saw
 * Arata's "Other Self"
 * Incomplete Girls

Trivia

 * In the 2012 anime, not all other selves have the exact same eye color as their human counterparts; Strength's eye color is orange instead of hazel like Yuu's eyes, and Saya has brown eyes while Black Gold Saw has red eyes. The rest of the other selves, however, have the same eye color as their counterparts.
 * In the cases of Strength and Black Gold Saw, however, their eyes are within the same chromatic scale (hazel is a darker and slightly less bright orange, and Saya's eyes are a muted brown, which can be considered be a darker version of Black★Gold Saw's red eyes).
 * It is revealed that the dead other selves can be revived as seen in Let's Go Beyond This World, where Chariot and Dead Master come back to help Mato alongside Black Gold Saw, and Strength coming back to life as well in the post-credits scene of the episode, despite dying earlier. How this happens remains ambiguous, but it appears that the act of their respective human counterparts remembering and coming to terms with their forgotten feelings and emotions plays a significant role.
 * In the same vein, a revived other self seemingly retains some memory of their previous life; upon encountering Black★Rock Shooter in the post-credits scene, the reformed Strength lowers her Ogre Arms and even bears the slightest hint of a small smile, as though implying that she no longer regards Black★Rock Shooter as an enemy.
 * Some of the known other selves possess demonic-like characteristics, such as horns, claws, and tails. Black★Rock Shooter and the hooded figures are the only other selves not to possess any of these characteristics.