Sayara 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, leaving her in the hands of other 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 had allowed her to walk again, but then her curiosity about the nature of the fruits, of Yggdrasil and why only women become immortals and men become Angel had once again drawn 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. She also has a fetish for piercings, and used them to torture Rin, as well as being bisexual.
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 can.