Yuki Kaji, the voice actor who fears AI but built it anyway

Yuki Kaji

Photo: ダイヤモンド公式チャンネル / CC BY 3.0, via Wikimedia Commons

Born	September 3, 1985 Tokyo, Japan
Occupation	Voice Actor
Known for	Weathering with You, My Hero Academia: Heroes Rising, My Hero Academia: Two Heroes
Awards	14th Seiyū Awards (2020) · Crunchyroll Anime Awards (2023) · Crunchyroll Anime Awards (2022) · 7th Seiyū Awards (2013) · 8th Seiyū Awards (2014)

The industry question Yuki Kaji hasn’t resolved — and hasn’t pretended to — is whether the AI voice system he built from his own recorded samples is a defense or a surrender. Bon Soyogi, his AI voice synthesis software, launched in December 2025, trained on his voice, and designed to produce output in both Japanese and English that no one can distinguish from a performance he actually gave. This from a man who told audiences that AI audio replication was “terrifying” — that the idea of his voice appearing in contexts he never approved, saying things he never said, was deeply disturbing. He used that word in public. He has not walked it back. He built the AI anyway.

Kaji grew up in Saitama, the prefecture north of Tokyo where he was born in September 1985, the older of two siblings. The move into voice acting started in 2004, when the industry looked very different — no streaming, no algorithmic discovery, no international fandom with real-time access to Japanese broadcast. He trained at Artsvision and accumulated smaller roles through the late 2000s, the kind of patient early-career accumulation that a field with few shortcuts requires. The Best Rookie Actor prize at the Seiyū Awards in 2009 was the first institutional signal that the work was registering.

His move to VIMS in October 2013 coincided almost exactly with the broadcast that changed the terms. Attack on Titan — 進撃の巨人, Shingeki no Kyojin — had aired that spring, and Kaji was Eren Yeager, the show’s lead: a teenager who begins terrified and becomes something the audience had to argue about for the next decade. The character’s transformation — from grief-driven child soldier to the wielder of a plan that most viewers found either tragic or monstrous — required Kaji to play the same person at radically different emotional registers across hundreds of episodes. He won the Best Lead Actor prize at the Seiyū Awards the following year. Then he won it again. Two consecutive wins for the same role in a field where competition for the award is fierce was not unheard of, but the specific task Eren Yeager set — sustaining emotional coherence across a character arc that climaxes in something most narratives would refuse — was harder than the prize category suggests.

What the decade after that demonstrated was that Kaji was not exclusively an Eren Yeager argument. Shoto Todoroki in My Hero Academia (僕のヒーローアカデミア), a character defined by a power he inherited from a father he resents and refuses to simply use, became one of the most-followed characters in global anime fandom — Kaji’s performance of emotional containment was central to the character’s appeal. Meliodas in The Seven Deadly Sins (七つの大罪) required a different register entirely: relentless energy, comedic timing, and the periodic switch into something much darker. Kozume Kenma in Haikyuu!! (ハイキュー!!) was quieter and more interior. Koichi Hirose in JoJo’s Bizarre Adventure (ジョジョの奇妙な冒険) occupied a completely different tonal world. Each of these would be a career centerpiece for most performers; together they represent something rarer — sustained creative range across concurrent productions across ten years.

By 2020, the 14th Seiyū Awards named him Voice Actor of the Year. Crunchyroll’s international audience voted him Best Voice Actor in the Japanese category for 2022 and again in 2023, both times substantially on the strength of Attack on Titan’s final season — a production that required Eren Yeager’s complete moral transformation to land with the weight of tragedy rather than cartoon villainy, which it did.

The complicated year arrived in October 2025. Kaji’s agency was forced to issue a public statement denying a wave of online posts claiming he had been involved in an extramarital affair. The content was, the agency confirmed, entirely baseless; legal action against defamatory posts was threatened and initiated. The episode was, in its way, a precise demonstration of what Kaji had been describing when he talked about AI voice dangers: content created and distributed without the subject’s knowledge or consent, intended to harm reputation through fabrication. The vectors were different — fabricated online posts instead of synthetic audio — but the structural problem was the same. A person’s identity, circulated as content they did not create, in contexts they never authorized. He had been warning about one version. He got the other.

In February 2026, Kaji announced he was leaving VIMS after thirteen years. The departure was clean and publicly cordial — no accusations, no stated grievance — and it aligned precisely with the launch of FRACTAL, the company he had founded and in which he serves as Representative Director, President, and CEO. FRACTAL’s stated purpose is creative collaboration; its central technical asset is Bon Soyogi. The framing Kaji has offered for the apparent contradiction — fearing AI voice replication while building it — is one of ownership: a performer who controls their own synthetic voice is in a different position than one whose voice is scraped, trained, and deployed without consent. Whether that distinction holds under market pressure, as the technology diffuses and competitors arrive with lower barriers to entry, is the question the next few years will answer. He is also co-creating a manga project with Hajime Isayama, the creator of Attack on Titan, a collaboration that closes a circle the original series opened in 2013.

Kaji married voice actress Ayana Taketatsu in June 2019. The couple announced the birth of their first child in November 2022. His family life stays largely out of public view, which is the way he prefers it.

FRACTAL has announced a planned album and live concert, extending the company’s scope beyond voice performance and AI software into music production. His most recent anime role is Kazuma Matsuoka in Blue Box, the 2025 series that gave audiences an entry point into his work that carries none of the weight of Attack on Titan’s decade-long arc. At forty, Yuki Kaji is not someone winding down a career. He is someone who has changed its terms, and who is still deciding what that means.