In-Birth （インヴァース, Invaasu） are individuals who control the EXS. In-Birth are people who are neither alive nor dead - they are people without a birth. Because of this, they are also called Fake Born (偽誕者, Gitansha).
During a Hollow Night, if a person gets bitten by a Void, it consumes the person's soul and infects their body with the Void's common ability to manipulate EXS. If a person can withstand their sanity during all this, they'll be fully awakened as an In-Birth.
In-Birth has the access to the Void's common ability, Manifestation (顕現化, Kengenka). It allows an In-Birth to give EXS some sort of unique shape. This is what's usually refered as an EXS ability.
In-Birth's body is called a Vessel (器[ヴェゼル], Utsuwa). It functions actually pretty similarly to a normal human body. Vessel is used to store EXS that a person needs to use their fighting ability. When an In-Birth's Vessel is pierced, they are briefly immobilized and are unable to use their abilities for awhile.