Atoko (also known as Atko) is an Atlach-Nachan. Her character design resembles Hatsune Hirasaka from the visual novel Atlach-Nacha; an outfit with a spider motif. (It is named after Clark Ashton Smith's creation Atlach-Nacha, the spider-god from the Cthulhu Mythos.) She has an appearance that is more mature than Nyaruko, Kūko and Hasuta, despite the fact that they are all around the same age.
Atko appears in the 10th light novel. Her family owes the largest Textile manufacturing company in space and she is on a trip to earth to do a research on a new material, and decided to stay in Mahiro's house. She was Nyaruko's classmate and friend in high school, and also Kūko's online friend that makes bodypillow with Nyaruko printing for her. Since high school, she has a bad habit of seducing other girls' boyfriends, as she felt pleasure when doing so, but dumped them once they are separated from their girlfriends. In novel she deliberately hugged Hasuta in front of Luhy to get pleasure from the latter's jealousy. She also states that she will support Nyaruko's pursuit toward Mahiro, and seduce him after they become couple so the guilt of robbing best friend's lover can bring more pleasure to herself. According to her, she can smell the scent of falling in love, and Mahiro has that scent, despite latter's denial. She fights with her own spider silk, web, and poison; according to Nyaruko, she is not as strong as them, but dealing with her attack is troubling.
In the Haiyore! Nyaruko-san F OVA episode, she has a cameo, having set up the escape game attraction that briefly robbed Nyarlko of her powers and memories, causing Mahiro to realize how much he loves her. She later sends Mahiro a letter explaining things, though his fuzzy memories of the event leave him confused.