Saibra was born with a mutant gene which caused her to shapeshift into any person she came into contact with. She came to view her power as a curse, as it caused people to distrust her since she would “look back at them out of their own eyes.” Saibra was thus alone even into adulthood. (TV: Time Heist)
Robbing the Bank of Karabraxos
As she was unable to control her ability, she helped the Twelfth Doctor rob the Bank of Karabraxos in order to gain a gene suppressant which would remove her powers. During the mission, she briefly impersonated Clara Oswald to demonstrate her power and impersonated a businessman named Mr Porrima to facilitate entering the bank before being cornered by the Teller. To prevent a painful death, Saibra activated one of the atomic shredders left by the Architect, apparently killing herself, followed by Psi soon after.
Saibra later returned with Psi, copying a security guard to rescue the Doctor and Clara after they were captured. The two revealed that the shredders had actually been teleports connected to a ship in orbit and aided in the rescue of the Teller and its mate. After the mission was successfully completed, Saibra shared a takeaway with the others before being returned to her own time and place. Having used the gene suppressant, Saibra was able to hug the Doctor without turning into him, something that she greatly enjoyed. (TV: Time Heist)
Due to a mutant gene in her physiology, Saibra had the ability to mimic the appearance of any life form she came into physical contact with right down to their DNA by replicating living cells. She wore a hologram shell to replicate clothing. The image could be maintained for as long as she needed. However, she had no control over her power and usually wore gloves to suppress it. Saibra later lost this ability due to the gene suppressant recovered from the Bank of Karabraxos.
As one of the people she had met during her travels with the Doctor, Saibra was included in a series of notes written by Clara when she was planning to confess to Danny Pink via phone call about her adventures with the Doctor. (TV: Dark Water)