Yūsaku Igarashi is a Japanese light novel author recognized as the original creator of several series that have been adapted into anime, most notably the Haruka Nogizaka's Secret franchise. His professional career began after winning the fourth Dengeki hp Novella Prize in 2003 for his novella Shiawase Nisei Taidōkyo Keikaku: Yōsei-san no Ohanashi. He made his formal debut the following year with the light novel series Nogizaka Haruka no Himitsu, which was published under ASCII Media Works' Dengeki Bunko imprint.

Igarashi is best known as the author of the Nogizaka Haruka no Himitsu series, a romantic comedy that follows the relationship between an ordinary high school student and a seemingly perfect classmate who secretly harbors a deep passion for otaku culture. The original light novel series ran for 16 volumes from 2004 to 2012. Its popularity led to multiple anime adaptations, including the 2008 television series Haruka Nogizaka's Secret, the 2009 second season Haruka Nogizaka's Secret: Purezza, and a four-episode original video animation series titled Nogizaka Haruka no Himitsu: Finale, released in 2012. In addition to these anime works, the franchise also spawned a manga adaptation, video games, and drama CDs.

Beyond his flagship series, Igarashi has maintained a consistent career as a light novel author with several other romantic comedy series. These include Hanikami Triangle, which ran for seven volumes from 2005 to 2008, and Koharubara Hiyori no Ikusei Nikki, a five-volume series published between 2009 and 2012. More recent novel works include Nogizaka Asuka no Himitsu, a spiritual successor series that began in 2018, and Chitchakute Kawaī Senpai ga Daisuki nano de Ichinichi Sankai Tera setai, which started in 2020.

His creative work has expanded beyond novels to include manga serialization and video game scenarios. Igarashi is credited as the original author for the manga series Sake x Koe Fleur, and he has contributed as a scenario writer and series composer for several games, including Blue Reflection: Maboroshi ni Mau Shōjo no Ken in 2017, as well as scenarios for titles such as Engage Princess and Shamei Echoes. His involvement in anime production has extended to writing scripts for drama CDs and the OVA series Nogizaka Haruka no Himitsu: Finale, in which he also made a voice cameo appearance as himself.

Igarashi's artistic identity is firmly rooted in the romantic comedy genre, often centering narratives on seemingly idealized heroines who possess hidden, relatable interests. His work on the Nogizaka Haruka no Himitsu franchise in particular established him as a significant figure in the late 2000s anime and light novel landscape, contributing to a popular wave of otaku-themed romantic comedies. His ongoing activity across novels, manga, and game scenarios demonstrates his sustained presence in the industry since his debut in 2004.