Tarvos' initial existence was rather short-lived. At first unsophisticated, a rough extension of Morganna designated for the sole purpose of eliminating her enemies, she did not have a real consciousness or purpose outside of that. Upon her defeat her remains sunk to the bottom of The World, until they were eventually recaptured by Amagi for Project RA. However, before she could be reinitialized for the Project, her inactive data was removed and hidden away on the PC of Jun Banshoya. This data was recovered by a woman named Reiko Saeki, and used to make a character named Pi.
Tarvos finally reawoke after Pi had a run-in with some AIDA. Rather than being her own individual she was attached to a PC - and the PC of a strong-willed individual. Tarvos continually found herself repressed, and even when she was allowed to surface was controlled quite tightly. It was a constant battle of wills between the human and Avatar every time, but Tarvos fought strongly and found herself getting closer and closer to winning. After that the frequency of Tarvos' use began to decrease sharply until she was only rarely called upon. Tarvos went into a somewhat dormant state, hating her imprisonment and inability to act upon what she felt was her true purpose. Her connection to Pi during this period of time resulted in the creation of her dual natures: the clarity of the logical human mind, and the infinite rage of the Avenger. The pollution of what she felt was her "pure state" only served to anger Tarvos further.
When Pi was infected with AIDA, Tarvos awoke for the first time in what felt like forever. However, she didn't truly get to stretch her limbs - rather, a clumsy foreign entity was controlling things. With some embarrassment, she stood by while her brother Skeith had to "rescue" her - something she did not take lightly. Before she could do anything else, she was forced back into dormancy, more upset than before. She had been made a fool of, and she would never forget.
When the Rebirth occurred, Tarvos was finally freed. She did not approve of the shape she was stuck with, but it was better than being locked away and unable to act. Finally reunited with her siblings, she felt they would be able to engage in their mutual purposes without hindrance. Unfortunately, she found herself unable to stand most of them: Skeith was too whiny; Innis was too flaky; Magus didn't understand the concept of PERSONAL SPACE; Gorre was too juvenile and had some personality issues that needed to be sorted out; Macha's animal thing was kind of creepy; and Corbenik, as the last of Morganna's children, didn't get enough hugs in his life and was showing it. Only Fidchell was slightly tolerable, and she'd listen to him rattle off lame prophecies in rapt attention. She grew closer to her older brother, so while the others were busy getting killed or falling off cliffs or self-flagellating or whatever else they were doing in their spare time, she would hang out with Fidchell and ignore them.
And then Fidchell went off one day to prophecize and Tarvos looked around and realized that everyone else had their share of epic drama that she didn't particularly care about and had all gone their separate ways. Rather than being stuck there all alone she decided in irritated youngest sister fashion to catch up to them (and kick them in the shins - if by kick you mean STAB WITH NAILS). Family is family, after all.