Tappei Nagatsuki is a Japanese novelist and screenwriter born on March 11, 1987. He began his writing career by publishing works online, initially gaining recognition for his web novel series Re:Zero − Starting Life in Another World, which he started serializing in April 2012 under the pen name Nezumi-iro Neko on the user-generated content site Shōsetsuka ni Narō. The success of this web novel led to a print publication deal with Media Factory in 2014, launching his career as a professional light novel author.

Nagatsuki is best known as the original creator of Re:Zero − Starting Life in Another World, a franchise that has expanded into multiple manga adaptations, anime series, and video games. He was actively involved in the production of the Re:Zero anime, attending script meetings, recording sessions, and dubbing. Beyond Re:Zero, his notable original works include the anime series Warlords of Sigrdrifa in 2020, where he served as series composition and screenplay writer, and Vivy: Fluorite Eye's Song in 2021, for which he was credited as creator and screenwriter. In 2024, he worked as script supervisor and screenwriter for Suicide Squad Isekai.

His involvement in the Isekai Quartet franchise stems from his role as the original creator of Re:Zero, one of the four isekai series featured in the crossover comedy anime. He is credited as original creator for Re:Zero across Isekai Quartet, Isekai Quartet 2, Isekai Quartet 3, and the film Gekijōban Isekai Quartet ~Another World~. The series brings together characters from Re:Zero, Overlord, KonoSuba, and The Saga of Tanya the Evil in a chibi-style school setting.

Nagatsuki has also contributed to other media, serving as scenario supervisor for the video games Re:Zero − Starting Life in Another World: The Prophecy of the Throne and Re:Zero – Starting Life in Another World: Witch's Re:surrection. He worked as a screenplay assistant for the theatrical film Pokémon the Movie: Everyone's Story in 2018. Additionally, he has written novel adaptations and side stories related to his original works, including Vivy Prototype and novels within the Warlords of Sigrdrifa franchise.

His creative identity is strongly tied to the isekai genre, though his works often explore psychological depth, character suffering, and complex narrative structures. Prior to his professional writing career, Nagatsuki worked as a meat processing handler, and he has stated that he wrote during his free time after working ten-hour shifts. He maintains friendships with several contemporary isekai authors, including Natsume Akatsuki of KonoSuba, Kugane Maruyama of Overlord, and Carlo Zen of The Saga of Tanya the Evil, which facilitated the creation of the Isekai Quartet crossover series. Nagatsuki has expressed particular fondness for the character Emilia from Re:Zero, noting that the series began from his desire to write a story about a silver-haired heroine.