Masayuki Kawaguchi is the original creator of the story that inspired the anime film Rainbow Fireflies. He began his career as a novelist, making his debut after finding success with a story he first shared online.

Kawaguchi started serializing his novel Rainbow Fireflies: Eternal Summer Vacation on his personal website in 2004. The story received positive feedback from readers, leading to its publication by the AlphaPolis publishing house in 2006, which marked his formal debut as an author. The novel was later adapted into an animated film titled Rainbow Fireflies, which was released in 2012.

Beyond Rainbow Fireflies, Kawaguchi has written other novels. His subsequent published works include Karakuri Yume Tokei DREAM∞CLOCKS, which was released in 2008, and UFO ga Kureta Natsu, published in 2013. All of his known published works have been released through AlphaPolis.

A significant personal event in his life was the 2011 Tohoku earthquake and tsunami. Kawaguchi experienced the disaster firsthand; his apartment and his family's clock and jewelry store in his hometown were destroyed. He and his family survived the event. The 2012 theatrical release of the Rainbow Fireflies animated film occurred in the aftermath of this disaster.