Yasuyuki Kosaka is a Japanese manga artist best known as the creator of the fishing-themed series Diary of Our Days at the Breakwater. His career as a manga author began with a one-shot version of this work, which was published in the first issue of Young Champion Retsu magazine in 2016. The series proved successful and began regular serialization in the same magazine starting with its third issue in February 2017. Kosaka serves as both the writer and illustrator for the series, which follows the activities of four high school girls who bond over recreational fishing in a coastal town.

Prior to establishing himself as a manga creator, Kosaka worked in the anime industry as a concept designer. One of his earliest known credits in this capacity was for a short anime titled Dragon Dentist, which was released as the first episode of the Japan Animator Expo in 2014. This work represented his initial involvement in animation before transitioning to a primary focus on manga.

Diary of Our Days at the Breakwater stands as Kosaka’s most significant and defining work. The manga, published by Akita Shoten under its Young Champion Retsu imprint, has been collected into numerous tankōbon volumes. Its popularity led to multiple adaptations. A 12-episode anime television series produced by the studio Doga Kobo aired from April to September 2020, with the broadcast experiencing delays due to the COVID-19 pandemic. Following the anime, a live-action web drama adaptation was also produced, airing from June to August 2023. The manga itself experienced temporary publication halts, including a hiatus in July 2020 due to severe flooding in Kosaka’s home region of Kyushu.

In addition to his primary series, Kosaka contributed to the manga anthology Kemono Friends: À la Carte - Japari Park Edition. His artistic identity is closely tied to the slice-of-life and sports genres, with a particular focus on depicting the intricacies of recreational fishing and rural coastal life. His work on Diary of Our Days at the Breakwater is noted for its detailed illustrations of fishing equipment, marine life, and the serene seaside environment, blending educational elements about fishing techniques with character-driven storytelling.