Carmodi Litian was one of the Unnoticed's time-sensitives. They bred time sensitives to alert them of the presence of time travellers or planets with time travel capability, as that could be a threat to their paradoxical existence.
As she got older, Carmodi could no longer endure her treatment and decided to commit suicide. She did this by claiming there was a time-source on Porconine, then insulting one of the Unnoticed once they got there. Thrown off a cliff, she survived the fall. She lived on the planet for five months until being rescued by a survey team.
Carmodi was addicted to the particles that accumulated in time travellers, feeling nauseous if she was not near one. She kidnapped Fitz Kreiner on Lebenswelt in 4009 and altered his memories to fall in love with her, so she would have a constant source.
After leaving Lebenswelt, she and Fitz were captured by the Unnoticed on Antimasque and taken to Tent City. The Eighth Doctor used The Book of the Still to find its location and rescue them. When he touched Carmodi, the potential charge she had built up being part of the temporal closed circuit that created the Unnoticed was released. (PROSE: The Book of the Still)