Oracca is a Rusalka who was driven insane by years of subterranean imprisonment on the planet Sudra.  She can be found wandering the halls of Indi. She is referenced in the Note Oracca, and possibly My Lovely.
Oracca had been held captive by the Sudran people in a small room. Due to undetermined circumstances which even the Rusalki do not seem to understand, she became ill and listless. Veruska initially tried to aid her with dream world algorithms, but Oracca showed no signs of getting better. Veruska then pleaded with Eshinimma, the High Priestess of Sudra, to move Oracca to a larger chamber so that she could have "room to stretch," or at the very least, to allow Veruska to put Oracca out of her misery.  Alternatively Veruska's dreams worlds may have been the source of the illness since they involved killing Oracca repeatedly. 
Although Eshinimma did have Oracca moved, it seems that her condition did not improve. Whether or not Oracca is dreaming by the time Trace meets her is not elaborated on.
Even though Oracca remains mute, never once speaking to Trace, it can be assumed that she is at least aware of him and her surroundings, since she provides Trace with a convenient mode of travel through Indi.
Incidentally, in the game data, Oracca has 1000 Hit Points and her allegiance is "Friend".
Initially, Oracca can be found in the center of Indi. She is mute, and thus has no cutscene interactions with Trace. However, if the player jumps on top of her head, she will turn and scuttle along in the direction Trace was facing when he landed. This provides expedited transport across Indi. Jumping again can be used to stop and change her direction.