Keiji Hirai is a Japanese voice actor born on February 15, 1979, in Tokyo, Japan. He is affiliated with the talent management firm Aoni Production. Hirai began his path to a voice acting career by enrolling in the Aoni Juku training school, graduating from the 19th term of the Tokyo campus in 1999. He officially launched his professional career after joining Aoni Production in 2002.

Hirai has built a career characterized by performing a wide array of supporting and guest characters across numerous long-running and popular anime series. His roles are diverse, spanning humans, monsters, and other creatures. Among his many credits, he has voiced several of the characters mentioned in the query, confirming his involvement in these projects. He is the voice of Funato in the 2019 series Girly Air Force. He also voiced the character Nama-bake in the fifth installment of the classic horror series Gegege no Kitarō, which aired in 2007. In the sports anime Rowdy Sumo Wrestler Matsutaro!! from 2014, he provided the voice for the character Tatsunokawa-zeki.

The query also correctly identifies his role in the Marvel Disk Wars: The Avengers anime, where he voiced the villain King Cobra, as well as other characters like the monster Fin Fang Foom. Furthermore, in the long-running franchise One Piece, Hirai has voiced numerous characters over many years, including Ikaros Much in the Fish-Man Island Saga story arc. His credits also include voicing no-chan in the Hokkaido Television Broadcasting mascot-based anime on-chan and its sequel, Yume-Miru Anime on-chan.

Beyond these specific credits, Hirai has been a consistent presence in the industry, with roles in major franchises such as the various Dragon Ball series, including Dragon Ball Kai and Dragon Ball Super. He has also appeared in the long-running detective series Case Closed, known in Japan as Meitantei Conan, and the supernatural comedy Gintama. His work extends to video games as well, with notable credits including the role of袁術 (Gishu / Yuan Shu) in the Koei Tecmo Warriors series and roles in Persona 5 and Xenoblade Chronicles 2. Known for his adaptable voice, he often plays multiple different characters within a single series, demonstrating his versatility as a supporting actor.