I need to backfill here. Viletta Nu is the Knightmare pilot that Lelouch stole a Knightmare from back in the first battle.
She has no memory of the incident, but she does remember Lelouch from right beforehand.
She's the one who told Shirley to follow Lelouch.
Shirley shot her with Lelouch's gun, but she didn't die. She was rescued later. She saw that he was Zero.
Shirley, meanwhile, is having a crisis. The man she loves also killed her father.
This is not helped when Mao, another geass user whose power is mind reading, finds her.