Juli is one of M. Bison's "dolls," a collection of young women abducted by Bison's organization and brainwashed into becoming highly loyal, emotionless soldiers that could serve as bodyguards or assassins. First introduced in Street Fighter Alpha 3, Juli is one of three playable dolls in the game alongside Juni and Cammy. In her Alpha 3 story mode, she hunts down Cammy, whose brainwashing is wearing off, in order to eliminate her. However, Cammy managed to free Juli from Bison's control.
Super Street Fighter IV
According to T. Hawk's story mode in Super Street Fighter IV, Juli went missing after the events of Street Fighter Alpha 3. T. Hawk learns of Juli's location from Rose, but she also warns him that Juli is a shell of her former self. It is revealed that Juli has been living in a cabin with an elderly couple that have watched over her. She has lost her memory, as well as her ability to speak and emote, but she spends all of her time staring outside as though she's waiting for someone; perhaps T. Hawk, though only Juli knows for sure.
Initially, it was implied that the Doll that Thunder Hawk was sent to rescue was Noembelu, who was a member of the Thunderfoot tribe, as is Hawk. However, due to Noembelu not being a playable or fightable character within the game, Juli's sprite was used instead, confusing those who followed the story.
To reconcile the discrepancies, supplemental materials explains that Hawk was looking for both Noembelu and Juli; Juli, back when she was Julia, moved from her home in Germany to live with the Thunderfoot tribe, and was T. Hawk's girlfriend.