The "Underworld" (UW for short) is the main setting of the manga, beside real life. All the otherselves live in this realm, constantly fighting, which is to defend their emotional state in the real life.
UW is the secondary world where the story takes place. It is said to exist for an uncountable time, but had been discovered about 20 years ago. The only way to enter this place is using STX1; a pair of gloves. If it is on the user's hands, it can be activated by will, and carries the user to UW.
Every character who can enter UW has another form, which is named Otherself. The form's appearance depends on the user's emotions; for example, if the character is feeling trapped in the real world, the Otherself has chains. The user's pelt design remains the same, the only difference is that the colors lose a little amount of color, expect the eye color. It is unknown if a part of the Otherself's appearance can be decided by the user, as it is never said.
The strength that the Otherself can have is depending on the user's mental state. If the character in the real world receives a negative emotion, it can either weaken the Otherself, or make it more stronger.