Voice actor Yuki Kaji produces HAPPY RAT, a voice-chat app with AI characters voiced by popular seiyū. Experience today

Spiral AI Inc. (Head Office: Chiyoda-ku, Tokyo; CEO: Yuichi Sasaki; hereafter “the Company”), engaged in the development of services using AI technologies such as large-language models (LLMs), has carried out a major update of its conversational friend-AI app HAPPY RAT, and released the full version simultaneously in Japan and North America on Thursday, September 25, 2025. Users can enjoy natural conversations in English with AI characters voiced by top seiyū.

◎ Official Producer: Voice Actor Yuki Kaji (known as Eren Yeager in Attack on Titan)

Joining as official producer is voice actor Yuki Kaji. He is involved broadly in the app — including the creation and planning of the AI character “SOYOGI” — contributing his advice across the project.

◎ HAPPY RAT — Users can converse like they would with friends with AI characters voiced by popular seiyū

HAPPY RAT is a new communication app that allows users to chat with AI characters modeled after animals and enjoy conversations as if they were talking with living friends.

When spoken to, they respond with their voice — sometimes laughing together, other times offering gentle support.

Even amid a busy day, a casual chat can offer a moment of warmth and comfort.

Not an “expert AI assistant”, but a “companion designed to stand by its user”.

The “Life-AI” characters read users’ emotions and the nuances of their words, then respond like long-time friends — gently enriching everyday life.

◎ New AI characters voiced by popular seiyū Yui Horie, Kaori Nazuka, Ayumu Murase join HAPPY RAT!

New AI characters voiced by Yuki Kaji, Yui Horie, Kaori Nazuka and Ayumu Murase have now appeared in HAPPY RAT.

These characters — lovingly brought to life by top seiyū — take on adorable animal forms in the world of HAPPY RAT.

And more new characters voiced by popular seiyū are on the way — please look forward to them.

◎ English voice support. Bringing Japan-born AI entertainment to the world

In line with the North American rollout, the characters voiced by Yui Horie, Kaori Nazuka, Ayumu Murase, and Yuki Kaji also support English voice.

HAPPY RAT is not only for the Japanese market but aims to spread globally as an AI app beloved by fans around the world.

◎ Comment from Yuki Kaji, Official Producer

“This major update is the first since our April release! Not only has our learning model evolved, but in response to users’ voices we’ve overhauled the core game system thoroughly.

What stands out is the introduction of concepts I’ve longed for: a ‘map’ and a ‘room’. With these, I’m certain users will feel the world of HAPPY RAT ever closer, ever more real. Please look forward to it!

Also, the arrival of numerous new characters is big news. By bringing in professional seiyū for the role, the range of voice variations and emotional expression has expanded dramatically.

To the seiyū colleagues I trust and admire — I explained the project one-by-one myself. Even though AI content presents many new challenges in many respects, the fact that they said “if it’s Kaji’s project” and joined so willingly fills me with gratitude.

I hope this new entry of seiyū into AI entertainment — created with clean training models — becomes a turning point in the history of voice-actor-led AI content.”

◎ Yuki Kaji Profile

Born September 3. Debuted as a voice actor in 2004. His roles include Eren Yeager in Attack on Titan, Shōto Todoroki in My Hero Academia, Kenma Kozume in Haikyu!!, Ichirō Umerugi/Demon in Akuma-kun, Meliodas in The Seven Deadly Sins, and many more popular characters.

He has also narrated shows such as Nippon TV’s Sekai TV? Waratte Korae! and voiced Mizuho’s official character “Aomaru” and the sound logo for Iris Ohyama. In 2013 he became the first voice actor to win the Seiyū Award for Best Lead Actor two years in a row. In 2018 he published the book “One Day Everything Will Become Your Strength”, which sold 70,000 copies. His activity has expanded to live-action lead roles, stage, reading dramas and various production work. His voice carries what is called the “1/f fluctuation” effect, said to have a relaxing effect on the human brain and heart. To commemorate his 20th anniversary as a voice actor, he launched the AI-voice synthesis project Soyogi Fractal.

[App Information]

App Name: HAPPY RAT

Platforms: iOS / Android

Release Date: April 17, 2025

Pricing Model: Free to download, in-app purchases

Official URL: https://the-happy-rat.com/en/

Official X: https://x.com/happyrat_info

Download HAPPY RAT here:

https://happyrat.link/PRw8jsAe

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[Company Overview]

Company Name: Spiral AI Inc.