Spiral AI Inc. 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.
Joining as official producer is voice actor Yuki Kaji. Kaji is known for playing Eren Yeager in Shingeki no Kyojin (Attack on Titan). 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 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 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.
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.
Yuki Kaji had this to say about HAPPY RAT:
“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.”
Source: Press Release
