Itsuki serves as Sensui's right-hand man in executing Sensui's malevolent schemes. Despite possessing formidable psychic powers, he requires another ability to tear dimensions, which is where Kuwabara comes into play. Kuwabara is the only one in Ningenkai capable of such a feat.
The story of how Sensui and Itsuki met involves Itsuki, then a youkai, attempting to fight and defeat Sensui. However, he failed and found himself at Sensui's mercy, as he was among Sensui's demon targets. Before facing execution, Itsuki was granted one wish. He wished for one more day to watch the final episode of his favorite TV show, which coincidentally was also Sensui's favorite. This shared interest led them to become close companions.
Throughout the series, it's evident that Itsuki is deeply interested in Sensui, closely observing his every move, including his emotions, weaknesses, and fighting skills (Absolute Anime).
