We all know Gai Amatsu. His arrogant attitude, his facade charisma, his underhanded method. We all hate him, he got Rider Kicked almost every Rider in Zero-One 1000%. And now we shall dive deeper inside his tragic childhood which led Gai become a villain.

Early life

Gai Amatsu was born on October 10, 1974 and grew up in a strict household. When he was 11, he owned an AI dog named Thouser, created by Hiden Intelligence, but his harsh father was angered by his 99 percent test score and forced him to return it to secure his education.

(A/N: Thouser)

This is Gai's Father, Ichikyo Amatsu

As the father of Gai Amatsu, he was very strict with his son as he was disappointed at Gai for his nearly perfect score of his exam due to his son's obsession with his robot dog "Thouser". Even though Gai studied hard to get his exam with perfect score, he was still mad to Gai about his 99 percent test score and he demand Gai to return Thouser.
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35 years later, after Gai became president of both ZAIA Enterprise Japan & Hiden Intelligence, he was in the middle of a downward spiral before having a conversation with the AI companion, Ai-chan. Suddenly, Zea responded by recreating Thouser with the 3D Printer.

The new Thouser was activated and greeted it's master after scanning Gai's emotions, the latter immediately recuperating Thouser's affection.

Thouser then join in the battle with Kamen Rider Thouser & Zero-One against Ark-Zero.

That's basically it, Gai had a sad childhood past because of his father who is a dick with a demanding attitude took away Thouser from Gai and Gai became villain, the rest is that we already know what he did when he became villain. But thanks to Zea who recreated Thouser now Gai has become an ally with an instant 180 degrees change. And yes he apologize to Aruto, Isamu, Yua.