The dark manikins offer their assistance in protecting the dark Crystal Core. Materia's edits to her champions' memories range from certain isolated events to large continuous swathes of time (for example, Cecil Harvey did not recall ascending Mount Ordeals or anything he did as a Paladin). Materia chose to do this in the belief that her champions could not achieve true respite if they were burdened by painful memories. However, most of them disagreed.
The dark manikins carry these excised memories in a form they refer to as "brilliance," with the manikin of each champion housing that person's memories. Usually, the manikin releases the brilliance to their original after being defeated in combat, whereupon the recipient has access to their complete memory once more. While characters generally recover all their memories after acquiring the brilliance from their respective manikins, it doesn't appear that the brilliance always carries a character's full memory as seen in the case of Lightning Saga characters where Noel appears to have memories from further point in time than Lightning and her companions, sans Serah.
After some time since the Blackened Will's defeat, Kefka imbues dark manikins of Terra and Celes with its energy, causing it to react to the brilliance stored in them and resulting in the rebirth of the Blackened Will. Kefka also steals a considerable amount of brilliance from other dark manikins, using it to enhance his power and abilities.