A Spirit is one of the Laments from Orpheus. While the term can be exchanged with others, such as ghost or post-life entity, the term spirit usually refers specifically to ghosts with a strong, independent presence who work for a projection firm such as the Orpheus Group.
To simply die is no guarantee of becoming a spirit. Many spirits, while alive, had multiple near-death experiences or have a strong connection with the living world they have yet to resolve. Because of a strong desire to remain connected with the living world and to not move on, the souls of these people stay instead of moving on to their next existence, a process called resolution or Transcendence.
Spirits who work for projection firms are especially prized, as they can carry out illegal operations with no fear of legal repercussions. Of course, such activities also put spirits at great risk, and in some cases, cause them to accumulate Spite rapidly, which can quickly turn a spirit into a Spectre if the agency isn't careful.
Spirits have one major disadvantage: an evil counterpart called a Doppelganger, which in essence is a Spectre counterpart with all of the knowledge and abilities of the spirit. Doppelgangers can be temporarily banished, but never totally destroyed, and usually provide their spirit counterparts with much grief and difficulty as they attempt to destroy everything the spirit tries to save. Spirits can usually sense the general presence of their doppelganger if they are close enough to it.
When a spirit becomes a Spectre, the doppelganger hunts them down and merges with them, completing the process. A redeemed Spectre, or an Orphan-Grinder, will split from their darker half again, but it becomes a special, more powerful type of doppelganger called a Gemini. Orphan-Grinders who again succumb to darkness will merge with their Gemini as they did before with their doppelganger. However, the Gemini forms a permanent tether with Grandmother, which erases a spirit's memories completely, making them into little more than a puppet that does her will.