Keiichi Sumi attends Mitsuhashi University as an upperclassman and friend to Misaki Takahashi. Their acquaintance began during Misaki's first week when Keiichi rushed to help after Misaki fell in a lecture hall, promptly inviting him to sit together. This sparked regular dinner invitations and consistent campus companionship.

Keiichi possesses long gray hair, which shifts toward grayish-brown in later seasons, brown eyes, and distinctive oval thin-framed glasses. As the son of renowned novelist Ryouichi Sumi, he originates from a literary family yet personally disdains literature, deeming novels a waste of time despite his father's prominence. He instead pursues economics academically.

He actively participates in Mitsuhashi University's Onsen Research Group (ORG), an organization historically recognized for creating Japan's most terrifying haunted houses. While his early attentiveness to Misaki suggested romantic interest, Keiichi's true motivation involved leveraging the friendship to access Akihiko Usami, a famous novelist and Keiichi's idol. This culminated in an attempt to seduce Usami while Misaki slept at Keiichi's residence, an effort thwarted by Misaki's intervention.

Despite this deception and subsequent rejection by Usami, Keiichi maintains a genuine friendship with Misaki, demonstrating ongoing concern for him. Following graduation at the start of Season 3, he secures employment at a video company after failing to enter the publishing industry. His name combines "noble" (圭) and "one" (一) for Keiichi, while Sumi (角) translates to "corner".