Rei appears to be a kind, but clumsy nurse who cares about her patients.

Spoiler

However, she truly sees her patients as "parasite hotels" for the parasites she is nurturing. She was fanatically obsessed with parasites, which she called "my everything" and her beloved. She would insert parasite eggs into patients having surgery at the hospital where she worked, and would later murder the patients to harvest the hatched parasites. Her own body was also heavily infested with parasites. After her crimes were discovered by Takumi, parasites crawled out of her body and formed a gigantic parasite several stories tall.

(Source: Danganronpa Wiki)