Speaking on Japanese television, Kyogo credited former Celtic team-mate Tomoki Iwata, who is also now at Birmingham, for selling the project.
He said: "I was happy because I received many good offers, but I decided Birmingham City's was the best for me.
"Tomoki Iwata contacted me and told me he'd be really happy if I joined him at Birmingham.
"I listened to what he had to say about the club and I got really excited about hearing about their ambitions."
Asked about his brief Rennes stint, he replied: "It's true that I had a difficult period at Rennes, but I don't regret going there.
"I think I grew in some ways, so I believe I can make use of that experience in Birmingham and I hope to make the most of it."