When a Gabranth awakens their bracelet, he'll form a contract with a Spirit which will grant the bracelet a name. However, even if they say they form a contract, the Spirit contracted are not those that are normally present in this world. The Spirit is contracted are those that lay dormant within the contractor. The nameless bracelet is the trigger to wake up that spirit.
Simply put, the Spirit is just another form of the user. A soul that existed in the past, exists even now without and end. However, only a few Gabranth are able to do something like this.