Ken met Yuuri in middle school, where they were in the same class. Despite going on to different high schools, Ken still serves as the manager of Yuuri's baseball team, and the two get together on a regular basis. Ken is known to smile often and keep relatively quiet. He later finds himself involved with Yuri and the other world when it is discovered he houses the soul and memories of the Great Sage. Dr. Rodriguez was responsible for handling his soul, around the same time when Conrad transported Julia's soul.

Ken is very smart and calculating, and capable of defending himself. He was the Great Sage in his past life - the right hand man, tactician and strategist to the Shinou, the Original King of Shin Makoku. He still remembers things from that life and other lives that he has had. He is a good friend and ally to Yuri and helps him a lot throughout the series, though it often seems he's working on his own agenda. He even constantly repeats to the Original King that he "likes Shibuya", and won't let anything bad happen to him.

Ken was actually the reason Yuri was given a swirly, since he was the kid the bullies had been picking on. He is always around when Yuri travels between worlds and later on in the anime travels together with him, he actually helps get Yuri to and from worlds although Yuri does not know this for most of the anime until Ken lands up in Shin Makoku with Yuri.

Later on, near the finale of Series 2, Ken seemed to have "betrayed" Yuri and his friends by not telling them vital information concerning the keys and what he had anticipated on what would come to pass. However, what he did was needed to help free the Original King. He is also seen scolding the Original King in his past life and normal life now, and sees himself as sort of a babysitter for the Original King.