Reiji Miyajima is a Japanese manga artist and writer born on December 15, 1985, in Nakano, Nagano Prefecture. He began his professional career in 2005 after receiving recognition in two major newcomer awards: the Magazine Grand Prix Encouragement Award for his work Sakka no Tensai, and an honorable mention at the 75th Weekly Shonen Magazine Newcomer Manga Award for his one-shot Pool no Saboten. Following these early achievements, Miyajima moved to Tokyo to pursue manga full-time, briefly working as an assistant to Yuji Terajima, the creator of Ace of Diamond. He also enrolled at Yoyogi Animation Gakuin to formalize his training in manga production.

Miyajima made his serialized debut in 2009 with Suzuki no Shiten, a short series published in Weekly Shonen Magazine. His first major long-running work began in 2010 with AKB49: Renai Kinshi Jorei, a comedy manga based on the popular Japanese idol group AKB48. The series, which ran until 2016, was written by Motoazabu Factory and established Miyajima as a reliable creator within the romance and comedy genres. In 2016, he launched Mononote: Edo Shinobi Kagyō, a historical action series set in the Edo period, which concluded in 2017.

Miyajima achieved widespread recognition with Rent-A-Girlfriend, which began serialization in Weekly Shonen Magazine in July 2017. The romantic comedy follows a college student who rents a girlfriend through a mobile app, and the complicated relationships that develop from this arrangement. The manga became a commercial success and received a television anime adaptation that premiered in 2020, followed by multiple subsequent seasons. Miyajima has also written a spin-off manga titled Rent-A-(Really Shy!)-Girlfriend, which focuses on a supporting character from the main series.

In 2022, Miyajima began serializing The Shiunji Family Children in Young Animal, a seinen magazine published by Hakusensha. The series explores family dynamics and forbidden relationships among seven adopted siblings, and received an anime adaptation that aired in 2025. His move to Young Animal marked a notable shift from Kodansha's Weekly Shonen Magazine, where he had published his previous works.

Beyond his manga series, Miyajima has contributed to multimedia idol projects. He served as series composition writer and character draft creator for the 2020 anime 22/7, a project centered on a virtual idol group. He also wrote the original story for its manga adaptation, 22/7 +α. These roles demonstrate his versatility as a creator who works across both print and animated media.

Miyajima's artistic identity is rooted in romantic comedy and relationship-driven narratives, often exploring themes of emotional vulnerability, social anxiety, and the tension between public performance and private feelings. His works frequently examine how people navigate romantic and familial bonds under unusual or artificial circumstances, such as rental relationships in Rent-A-Girlfriend or forbidden sibling dynamics in The Shiunji Family Children. He is known for working primarily with digital illustration tools, a method that allows for efficient production and remote work. Within the industry, Miyajima is recognized as a commercially successful creator whose works have consistently received anime adaptations, solidifying his position as a notable figure in contemporary romantic comedy manga.