The Sixth Neck (六の首, Roku no kubi), she is a small, squeaky-voiced catgirl (her name is derived from neko, the Japanese word for cat) who is often seen wearing a nurse's outfit and wielding a giant syringe.

Spoiler

She appears to fit the stereotypical, cheerful catgirl archetype, but it is later revealed that she was the subject and victim of horrible medical experiments, which is most likely what made her an Orochi follower. Nekoko's syringe contains some form of medication that can heal injuries and sickness, but the injection causes horrible pain, as noted by Girochi.

(from wikipedia)