Other self

An "other self" is an alternate counterpart to a human character that resides in the Otherworld. These "other selves" possess an invisible connection with someone from the human world. They are subconscious beings who have the ability to defend themselves using their own weapons against the monsters they spawn, skyrunners, whenever their human counterpart feels a negative emotion.

Description
An "other self" has some sort of connection or personality similar to that of their human counterpart. Under certain circumstances, presumably the "other selves" can create a connection to their counterpart and make a deal with him/her to "merge" or "fuse. It is rare to extremely unlikely that an other self can leave the Otherworld and inhabit their real self body, though if they were to do so, they would leave the Otherworld entirely, whereas the human counterpart's soul has to leave their body behind to do so. One of the few ways to separate a soul from a body is death, or at least severe injury, of the human's body. As long as their working body is in the human world, the human can always go back. The only people who have been seen to merge their other self with their human counterpart are Ragner Blake, Jimmee Tynan, Jesse Tynan, Maloree Tynan, Ember White, Tamika Bennett, and Lucy Michaela, and it can be assumed that Ashton managed to do the same as well when he inhabited Jem's preserved body in the human world. After Reaper Devolution, Jesse's Other self, returned to his normal form, Jesse appeared as his human form, dressed in Reaper Devolution's clothing. While Jesse was using Reaper Devolution's body, his wounds did not heal as quickly as they would for "other selves" (although later in the battle he was able to withstand more damage, suggesting some healing ability) and his blood was red. This was not the case for Ember, who could withstand massive damage and had Emberlee's black blood. Her enhanced durability may however be attributed to her masochistic personality and the fact that she has the supernatural powers of a witch.

How much control each party has when merged is also unknown. When merged/possessing Jesse's soul in his other self's body at the end of Torment, Reaper Devolution was behaving in a violent, cruel manner, much unlike Jesse; however, it is possible Jesse had lost control or was sharing Reaper Devolution's angry instincts. However, Reaper Devolution seemed to be empathetically reacting to Elain Delmon in a way Jesse would be likely to.

The battles between other selves often reflect the experiences of their human counterparts in real time, as shown when Emberlee fights Molly at the same time when Ember and Maloree "fight" for Jeremy's affection. 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 human world would sometimes "bleed through" the Otherworld; human voices can be heard from the Otherworld, while a human's physical status may reflect in the Otherworld as well (for example, Mediah's robotic spine reflecting Jimmee's use of the wheelchair and the metal leg that Reaper Devolution and Jesse have). Likewise, events in the Otherworld may also manifest in the human world as dreams.

Maloree Tynan explains 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. The "other selves" are implied to be this, as Emberlee is seen screaming in pain as Ember goes through emotional turmoil.

Eventually, Ashton reveals to Jesse that these beings have essentially taken on the grief of each counterpart in the real world, but killing an "other self" causes the real counterpart to lose his/her attachments when becoming free of grief, including love. This occurs with the death of Jimmee's "other self"; Jimmee lost her love for Jesse and her will to live. Ashton disapproves of this way of dealing with grief, believing that differences should be resolved directly by talking. One particular "other self" of interest is Molly, who appears to have control over reality in the Otherworld itself, to some extent. She seemed to be able to stop the flow of the destruction in Jem and Ashton's world, to the point where it was like stopping time, and the blades that cut the fissure that carried Mediah away seem to cut into reality itself. It is later revealed that her duty, given by Ember, was to protect the Otherworld.

Trivia

 * Before Emberlee refers to Reaper Devolution by name, Emberlee refers to all of these beings as "the other [name]".
 * Other selves don't have the same eye color as their human counterparts, and are instead a bright and solid color, such as gold or purple. As an example, Ragnar was able to tell that it was Jesse was in his Other self's body by his "human eyes".
 * As the term itself is called "Other Self", all human beings have counterparts in the Otherworld.
 * It is revealed that killed "other selves" can be revived, as shown in Ashes where Molly comes back to help Ember along with the main "other selves" cast, and Ashton being alive and well in the final scene of Torment despite dying earlier. Partially how this happens is 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 Reaper Devolution in the epilogue of Torment, the reformed Ashton lowers his weapons and even bears the slightest hint of a small smile, as though implying that he no longer regards Reaper Devolution as an enemy.
 * Most of the "other selves" have black-colored blood, with a shade of the color of their eyes.
 * Regardless of their personalities, all "other selves" share one common characteristic - a deep and unconditional love for their human counterpart who they regard as the most precious person to them.