Shizuka Ijūin was the primary pen name of Tadaki Nishiyama, a Japanese writer and lyricist born on February 9, 1950, in Yamaguchi Prefecture. A graduate of Rikkyo University, he began his career as a commercial director for an advertising agency before turning to writing in 1988. He also worked under the pen name Ayumi Date, particularly for his work as a lyricist. Ijūin died on November 24, 2023.

Within the context of anime and manga, Ijūin is credited as an original creator. He is noted as the original creator of the manga Tenshi no Kajitsu, which was serialized from June 1993 to October 1994 and published in three tankoubon volumes, with art provided by Waki Yamato. In anime, he served as the original work creator for the 1997 movie Kikanshasensei. Additionally, under the name Ayumi Date, he was a prolific lyricist for various anime theme songs, including works for series such as Cat`s Eye, Eagle Sam, Kagaku Kyuujo-tai TechnoVoyager, and Shin Mazinger Shougeki! Z Hen.

Beyond his contributions to anime and manga, Ijūin was a highly regarded figure in Japanese literature. He made his literary debut in 1981 and received numerous awards, including the prestigious Naoki Prize in 1992 for his short story collection Ukezuki. Among his most celebrated works are the autobiographical trilogy Kaikyou and the bestselling essay series Otona no Ryugi. His body of work, which often drew from his personal experiences with loss, earned him a significant place in contemporary Japanese literature.