Yoriko Yasaka serves as a supporting character, mother to Mahiro Yasaka, and a monster hunter—a career she started in college, keeping it secret from her son for years. Her youthful appearance belies having a high school-aged child, marked by dark brown hair and grey eyes.

Kind and optimistic, especially towards friendly aliens, she welcomes Nyaruko, Kuuko, and Hasuta into her home despite their extraterrestrial origins, accepting their interest in Mahiro. While initially advising Nyaruko to slow down her pursuit of Mahiro, Yoriko later permits their relationship after they rescue her. She bonds quickly with Kuuko over a shared passion for video games.

Yoriko deeply loves her son, frequently expressing affection through hugs, which sometimes makes Mahiro uncomfortable. In combat, she primarily wields a fork, mirroring Mahiro's weapon choice, a style tied to her monster hunting background. Her name combines the Japanese words for "rely" (頼) and "child" (子), with her surname meaning "eight" (八) and "slope/hill" (坂).