Shiala was an acolyte of Matriarch Benezia for nearly two centuries, respecting her wisdom and biotic ability. Along with many of Benezia's followers, Shiala accompanied the matriarch aboard Sovereign as Benezia tried to guide Saren Arterius down a gentler path. When Benezia failed and became indoctrinated, Shiala also became one of Saren's thralls.
Shiala willingly serves Saren and accompanies him to Feros, where Saren encounters the Thorian and discovers that the ancient alien plant had the Prothean Cipher he needs to understand the vision from Eden Prime. He manages to convince the Thorian to accept a trade. Shiala is 'sacrificed' to the Thorian and absorbed by it, sharing a single consciousness. She then uses her biotics to gain the Cipher, which she transfers to Saren's mind. Once he has what he needed, Saren abandons Shiala to the Thorian and orders the geth to destroy it, hoping to prevent Shepard from gaining the Cipher.
When Shepard's team arrives, the Thorian is somehow able to create clones of Shiala which serves to communicate with Shepard. These clones are also biotics who attack Shepard's squad as they try to wipe out the Thorian. After destroying the Thorian, Shiala is freed and is fully herself again. She tells Shepard of Saren's actions and how Benezia became a slave to his cause, and explains about the Cipher. Knowing the vision is the key to finding the Conduit, Shiala touches Shepard's mind and directly transfers the Cipher. She then asks to be allowed to go to the Zhu's Hope colony and help them rebuild as penance for what she had done.
Shepard can then determine Shiala's fate — by releasing her to help Zhu's Hope. Alternatively, Shepard can determine that Shiala "switches sides too often" and thus decide to execute her; Shiala will accept her fate and kneel, offering no resistance as Shepard shoots her in the back of the head.
Shiala is the only individual who has managed to completely overcome Sovereign's indoctrination without resorting to suicide. This is likely because she was linked directly to the ancient Thorian and 'indoctrinated' by the plant being in turn. After the Thorian died her mind was her own again.
Mass Effect 2
If Shepard allowed Shiala to live after killing the Thorian, she can be found on Illium, investigating a medical contract made with the colonists of Zhu's Hope. Shepard can offer to negotiate with the company on Shiala's behalf. In the months after her exposure to the Thorian, Shiala's skin pigmentation changed to green — similar to her clones — and her biotic abilities became unstable. She also mentions disconcerting dreams of her time with the Thorian.
If Shepard helps her, Shiala also professes an interest beyond mere friendship, saying: "Maybe sometime, when I'm not organizing the colony and you're not... doing whatever you do." She then smiles and strokes Shepard's arm before leaving.
However, if Shepard allows the Feros colony to die (but allows Shiala to live after freeing her from the Thorian), then her dialogue and interactions with Shepard will change. She will say instead that she is a commando and doesn't know how to help people, but she will try. She will also not flirt with Shepard once they help her.
Mass Effect 3
If Shiala survived, Commander Shepard will receive an email detailing how the colonists on Feros are fighting the Reapers. Due to the Thorian spores, colonists are given an uncanny ability to sense one another, allowing them to think and act as one in battle. The long-term consequences of this connection are unknown, but helpful for the time being.
Shiala also mentions that she feels as though she is still indoctrinated, as she can feel the Reapers' influence in her mind. However, she says that this is overridden by her connections to the other colonists. She and the colonists contribute to the war effort.