Sayara first appears in Episode One when Rin tries to solve the mystery of Kouki's detached memories. She tortures Rin in her labs, but after Rin survives and completely heals her injuries she confronts Sayara and leaves her in the hands of her own delirious test subjects.
Sayara's story might have ended there if not for the intervention of Apos, who had given her a Time Fruit to survive. However the fruit did not heal Sayara's injuries so she spent the better part of 21 years dealing with them. In time, however, evolving technology allowed her to walk once again.
Her curiosity about the nature of the fruits of Yggdrasil and why only women become immortals and men become Angels once again drew her into conflict with Rin.
Sayara is a skilled bacteriologist, which she used to try to fathom the secrets of immortality from Kouki's D.N.A.. However this skill coupled with her great scientific curiosity often fuels her sadistic nature.
Sayara reappears in Episode Three where it turns out she had survived as an immortal but has to rely on a powered exoskeleton to move, mostly because of the limitations of the Time Fruits (anyone who is healthy at the time of possession remains healthy, while those who are ill or injured remain so). She also seemed to have a relationship with Yuria Takimoto. Her goal is to exterminate humans with Higan (a deadly disease native to Hagashima Island, also known as Death Island), leaving only immortals on Earth. She is finally killed when Apos consumes her Time Fruit before Kouki could.