Seimaru Amagi is a prominent Japanese manga storywriter, novelist, and screenwriter, best known as the original creator of the long-running mystery franchise The Kindaichi Case Files. He was born on July 22, 1962, in Tokyo, Japan, and is a graduate of the economics and political science program at Waseda University. Seimaru Amagi is a pen name; his legal name is Shin Kibayashi. He maintains a prolific career by working under several different pseudonyms for various projects, with other notable pen names including Tadashi Agi, Yuma Ando, and Yuya Aoki. The name Tadashi Agi is shared with his sister, Yuko Kibayashi, with whom he collaborates on some works.

As Seimaru Amagi, his most significant contribution to manga and anime is The Kindaichi Case Files, which began serialization in Kodansha's Weekly Shonen Magazine in 1992. The series follows high school student Hajime Kindaichi, the grandson of famous detective Kosuke Kindaichi, who solves intricate murder mysteries often involving locked-room puzzles and seemingly impossible crimes. While the earliest cases were written by Yozaburo Kanari, Amagi took over as the sole writer for the series in 1998 and has continued to write its many installments, including the File, Case, New, 20th Anniversary, and Return (R) series. The franchise has spawned multiple anime television series and films, including The File of Young Kindaichi, which aired from 1997 to 2000, the 1999 anime film The File of Young Kindaichi: Murder in the Deep Blue, and the 2014 and 2015 seasons of The File of Young Kindaichi Returns. The character of Inspector Kengo Akechi, a prominent figure in the series, received his own dedicated manga spin-off, The File of Inspector Akechi, also written by Amagi.

Beyond the Kindaichi franchise, Amagi has created several other notable manga series under this pen name. He wrote Detective School Q, serialized from 2001 to 2005, which focuses on a group of students at a prestigious academy training to become detectives. From 2002 to 2004, he created Remote, a series about a former detective who solves crimes from his home using a partner as his eyes and ears. His other works as Seimaru Amagi include Zero: Kage Miko, a horror manga adaptation based on the Fatal Frame video game series, Y no Hakobune, and the ongoing mystery series Gifted, which began in 2021.

The creator's artistic identity is defined by a masterful command of the mystery genre, particularly the Japanese "honkaku" (orthodox) school of detective fiction, which emphasizes fair play and presenting all clues to the reader. His stories are characterized by intricate plots, dramatic reveals, and a focus on the psychological motivations and tragic backstories of the criminals. His industry significance is immense, as The Kindaichi Case Files is one of the best-selling and most influential manga in its genre, helping to establish and popularize the mystery formula in shonen manga. The cumulative sales of the franchise exceed 100 million copies worldwide. His work under other names is equally noteworthy, including the award-winning Kunimitsu no Matsuri (as Yuma Ando), the action series GetBackers (as Yuya Aoki), and the wine connoisseur manga The Drops of God (as Tadashi Agi). His influence extends beyond manga, as he was also hired by Nintendo to write the story for the video game Fire Emblem Fates. In 2021, Netflix announced that he is writing an original anime series titled Lady Napoleon.