Kosuke Onishi is a Japanese voice actor born on May 7, 1991, in Saitama Prefecture. He is affiliated with the talent agency Arts Vision. Onishi's journey into voice acting was inspired by his experience playing the game Tales of Symphonia, which led him to pursue formal training at the Japan Narration Acting Institute.

He began his career in the early 2010s, securing minor roles in various television anime series. His early credits include appearances in productions such as Sword Art Online, Kingdom, and The Knight in the Area in 2012. Over the following years, he continued to build his resume with a wide range of supporting characters in shows like Food Wars!, Ace Attorney, and Berserk.

Onishi's role as the character Neinhart in the long-running fantasy series Fairy Tail is one of his more prominent performances. Neinhart is a member of the elite Spriggan 12 and a user of the magic Historia of the Dead. He also voiced the character in the 2024 video game Fairy Tail 2.

In 2020, Onishi took on the role of M-21 in the Crunchyroll original anime adaptation of the popular manhwa Noblesse. M-21 is a modified human and a former Union experiment who becomes a key member of the RK group. The announcement of his casting was part of the show's initial promotion. His role in Noblesse was highlighted alongside other main cast members, bringing the character to the screen for an international audience.

Beyond these roles, Onishi has demonstrated his versatility across many other popular anime series. His credits from the late 2010s and 2020s include appearances in The Ancient Magus' Bride, Vinland Saga, Tower of God, Jujutsu Kaisen 0, and Chainsaw Man. He has also worked extensively in video games, perhaps most notably as the voice of the popular Zora prince Sidon in The Legend of Zelda: Breath of the Wild and its sequel, Tears of the Kingdom. His work also extends to dubbing foreign content and narration.