Shirabe is a quiet and reserved, yet passionate and stubborn young girl. She is fiercely protective of who and what she holds dear, particularly Kirika. She claims to not be good at talking to other people, so she intentionally avoids getting too close to people who aren't Kirika.
At a very young age, Shirabe was caught in a traffic accident that took both her parents' lives, and she was the only survivor. Due to that accident, she lost most of her memories and any documents that explained who she was. Shirabe was then kidnapped by the F.I.S. and placed in the White Orphanage, as one of the Receptor Children who could potentially host Finé's soul. Her current name of 'Shirabe Tsukuyomi' was given to her by the FIS, as Shirabe could not remember how the kanji of her name was originally read; AXZ Episode 9 implies her first name was originally read as 'Tsuki'.
After the death of Ryōko Sakurai during season 1, the FIS attempted to determine if Finé had revived in any of the Receptor Children, but were unable to do so. They then planned to dispose of the project, but due to an armed uprising led by Nastassja — with the help of funding provided by the Bavarian Illuminati — the existence of the confined Receptor Children was brought to light. In fear of criticism from the international community, the US government quickly changed their tune and arranged for the children to be released of their own will or provided shelter. At this point, Maria, Shirabe, and Kirika joined with Nastassja and Dr. Ver in order to stop the fall of the Moon.
