When Grace was just 12, she was kidnapped by John Shepard for use as DAL's central processing unit. Because she was too resilient to allow any notable use of memory, Shepard implanted a virulent alternate personality in her. This allowed DAL to use her as a processing unit, but giving her severe mental damage.
Eventually, during the battle against Side 8, Klak accidentally triggered her release from the DAL unit. Because her virulent personality was possessed by the virus Uterio di Armechio planted in there, she attempted to kill Klak. However, User Zev appeared to intervene. After a brief fight, during which Feldt begged him to kill her, User Zev merged the two personalities fighting for control over her, allowing her to resume her life.
To repay him, she used her status as DAL's processing unit to initiate Side 8's autodestruct, removing one of Uterio's most powerful weapons... forever.