She is only fought in the Bad and Bad+ (non canonical) endings, and is the final boss in these endings.
With Alessa Gillespie complete once again with her other half, Cheryl Mason, the Incubator forms from the God that has matured in her body, creating the image of a beautiful, dark-haired woman. She glows with a heavenly light and wears pristine white robes.
If Harry Mason completes an earlier side quest to save Michael Kaufmann and find a vial of Aglaophotis, Kaufmann will arrive and throw the Aglaophotis on the Incubator, expelling the evil from her body in the form of the Incubus, which Harry will fight instead, resulting in either the Good or Good+ endings.
Despite her heavenly appearance, she is still a monster, just like the Incubus, and after her creation, she kills Dahlia Gillespie and attacks Harry. The Incubator fights and behaves almost exactly like Incubus (except for the fact that it takes less damage to kill the Incubator). She floats along the ground in a large room, completely unhindered by gravity. She attacks by firing blasts of heavenly lightning in volleys of three. As she does a large amount of damage, it is advised to keep moving until she stops her volley and then attack with a high-strength firearm.
It is possible to kill both Incubator and Incubus without dealing damage. If the player uses up all of his ammunition before the fight and survives for a few minutes, the boss will react as if the final shot has been fired. However, this will not work if the player has the Hyper Blaster.
In the Good or Good+ endings, the Incubator reappears after the Incubus's death. Her aura dims and reveals Alessa, who gives Harry a newborn baby. In the Good ending, she uses her power to save Harry and the baby by freezing the Otherworld as it collapses until Harry and the baby escape, but in the Good+ ending, she saves Harry, the baby and Cybil Bennett. She then dies after the overuse of her powers, and is consumed by flames.
The Incubator is Alessa's mental image of God, which is much more saintly and "pure" in appearance than the Incubus, which also symbolizes Alessa's ideals.
The Incubator also appears to be Alessa, only more adult-like. The Incubator's death (as Alessa) in the Good and Good+ endings may be also considered similar to the death of the God in the myths of the Order: she died due to overuse of her powers.