Masumi Hirai is a Japanese voice actor recognized for his extensive work in adult visual novels and anime, particularly within the boys' love and hentai genres. He began his voice acting career around 2003, initially using the stage name Masumi Hirai before adopting the more widely known pseudonym Tatsuya Hirai. Both names are aliases for the voice actor Daisuke Hirakawa, a practice common in the Japanese adult entertainment industry.

He is known for bringing to life a range of complex and often dramatic characters. A notable early role was as Kazuhiko Amamiya in the OVA series Taboo Charming Mother, where he voiced the rebellious stepson. In this production, he was credited as Masumi Hirai for the first three episodes and as Tatsuya Hirai for the subsequent three. This role is an example of his work in adult-oriented media, for which he became a memorable figure.

His career expanded significantly with major roles in hit titles across various media. He gained widespread recognition for portraying the controversial protagonist Makoto Itou in the School Days series, which included both anime adaptations and games. In the boys' love genre, he became a sought-after voice actor, delivering acclaimed performances as Ivan Fiore in Lucky Dog 1 and Katsuya Saeki in Kichiku Megane. His portfolio also includes roles in otome games, such as Mizuto Nomiya in Chou no Doku Hana no Kusari.

Throughout his career, Hirai has demonstrated versatility, taking on main and supporting roles across adult games, drama CDs, and character song projects. While he has not received major public awards, he is highly regarded by fans and critics within the visual novel and BL communities for his expressive voice and ability to convey nuanced emotion, adding depth to the characters he portrays. He maintains a private personal life, focusing public attention on his extensive body of work.