Yūki Yaku is a Japanese light novelist, manga writer, and screenwriter born in 1991. Before establishing himself as a writer, he worked as a comedian and created videos during his late teenage years and early twenties. His professional writing career began when he won the 10th Shogakukan Light Novel Award in 2015 for a manuscript titled "Manten Kazari no Ganbari Ron!" which was subsequently retitled and published as his debut work, Bottom-tier Character Tomozaki, in May 2016 under the Gagaga Bunko imprint.

Yaku is best known as the original creator of the Bottom-tier Character Tomozaki series. The light novel, illustrated by Fly, follows high school student Fumiya Tomozaki, a skilled gamer who views real life as an unbalanced game until a popular classmate offers to help him navigate social situations. The series remains ongoing, with multiple volumes published since 2016. Its success led to several adaptations. A manga version illustrated by Eight Chida was serialized in Monthly Gangan Joker from December 2017 to February 2021, and a spin-off manga focusing on the character Minami Nanami, titled Minami Nanami Wants to Shine, was illustrated by Bana Yoshida and ran from July 2020 to January 2023. The property was also adapted into an anime television series; the first season aired from January to March 2021, followed by a second season, Bottom-tier Character Tomozaki 2nd Stage, which aired from January to March 2024. For these anime productions, Yaku contributed original screenplays for original video animation episodes included with the home media releases of the first season and also wrote scripts for the second season.

Beyond his original series, Yaku expanded his creative portfolio to include screenwriting for original anime. In 2024, he served as the series composer and scriptwriter for the television anime Jellyfish Can't Swim in the Night, an original production from animation studio Doga Kobo. He also wrote a novelization of the series, released in three volumes by Shogakukan under the Gagaga Bunko imprint in 2024, with the original anime credited as the source material under the collective name JELEE.

Yaku’s career demonstrates a trajectory from award-winning novelist to a multi-hyphenate creator involved in both writing source material and adapting it for the screen. His work consistently explores themes of self-improvement, social navigation, and applying strategic thinking to interpersonal relationships, often using game-like frameworks to examine personal growth and the contrast between idealized and real-world challenges. His significance in the industry lies in his successful transition from light novel author to a central creative figure in anime production, handling both original story creation and series composition for major anime titles.