Ikusu Mizutani is a significant character in the anime series 'Kono Naka ni Hitori, Imouto ga Iru!' She is a skilled and intelligent private detective who is hired to protect and assist the series' protagonist, Shougo Mikadono. Disguised as a male student under the alias 'Mr. X,' Ikusu provides crucial information and support to Shougo, helping him navigate the challenges of finding his true sister and choosing a suitable bride. Despite her serious demeanor, Ikusu often showcases a playful side, adding depth to her character and interactions with others.