Hikari Kongou

A shy but incredibly intelligent high school student in the Gifted Youth program and Kanau's younger sister. She calls Taihei her big brother despite them having no family relationship and sometimes visits his and Umaru's apartment for food.

[Spoiler] She is one of the few people aware that Umaru and Komaru are the same person, alongside Taihei and Umaru herself. [/Spoiler]

(Source: Wikipedia (edited/expanded))