Lana Volkov is the sister of Roman Volkov. Like her brother, she was kidnapped while their mother was at work and taken to an orphanage in Ukraine. There she and Roman were bought by Mercer, a crime lord who became interested in experimenting with IAAN on kids. She nearly died during the experiments as her health had been poor since childhood, but she eventually recovered and her powers manifested much later than Roman's and Priya's.
Lana's ability as a "Limit" was forced on her by Mercer. It allows her to negate other PSI's abilities. Her power has such force that she can use it on multiple people at once, as well as make people feel sick when under her control. Unlike Roman and Priyanka, there are no known side effects to her power. She is the only known character to have this power.
The Darkest Legacy
Lana has been brainwashed by Mercer, and is tasked with bringing her brother and Priyanka back to him.
Priyanka: former girlfriend
Lana and Priyanka became close during their time with Mercer, and eventually entered a relationship. Mercer did not allow this, and forcibly kept them separated at all times. After Roman and Priyanka escaped Mercer, Lana felt betrayed and grew to hate Priyanka. Towards the end of the book, they show signs of possibly getting back together, though it is not confirmed.
Roman Volkov: brother
Captured and experimented on together, the siblings' bond grew so strong that Roman would give his life in order for his sister to return to normal. Roman and Priyanka search for Lana throughout The Darkest Legacy.