Yuriev's Return
Citrine is the first person aware of Yuriev's return after he took over Gaignun's body, as she could speak to him through telepathy. She seems to start acting as a special agent for him, and soon to be his bodyguard; she also fill him with information as he has no information on the current situation.
Further Events
She acts as Yuriev's bodyguard in on year after he came back, as well as special agent doing some 'dity' work. She receives her orders from him, but stays in the shadow until the Durandal takeover, where she helps him to control the ship, especially by easilly defeating and controlling Canaan as he tried to kill Yuriev. When Yuriev will go to take control of the Zohar Emulators, she decide to stay behind to prevent anyone from stopping his plans, but especially because she knows Rubedo will come through this way.
Death
When Junior and the teams try to stop Yuriev, they encounter her as she stood to meet them. At this point, it appears she doesn't follow Yuriev's order for his plan in itself, but in order to destroy U-DO, the reasons for what she was born, even if it threatens the existence of the universe. She also reveal to hate Rubedo for what he did 15 years ago in the Old Miltia. She shows similarity with Albedo in that her main argument against Junior is to have abandoned them 15 years ago. Forced to fight her, Junior and the others defeat her; she questions what is their reason to exist at the moment of her death, the reason of her birth, and asks an answer to Rubedo, who is still alive even after having betrayed his comrades. He says he lives to keep Sakura's promise; to what she replies in saying she pities him before dying.
After her death, both Rubedo and Nigredo show sadness about it, and she is referenced for the last time after Nigredo's sacrifice by Albedo, who says that Nigredo, along with her, are probably going to spy on them.