A talented boy who transferred into the 6th grade of elementary School of Clamp School. He is very kind and good at sports and piano.
He resents Imonoyama Nokoru and seeks revenge against him. He challenged Clamp School Detectives in the battle of wits.

Although unwanted by the Yudaiji family, Idomu's mother was very dear to him. However he doesn't remember her smiling at him. One day when Idomu was playing together with Nokoru, his mother gave Nokoru a smile that Idomu wished to receive himself. That small event gave birth to the hatred towards Nokoru that Idomu have been piling up all those years. For stealing something dear to him, Idomu decided to steal something dear to Nokoru.