Shiranui (シラヌイ (白野威),Shiranui?) is the "white" kitten Hayate receives from Hinagiku. He is a black with a small white X-shaped scar in his forehead. He has a very devilish personality and deliberately gets Tama into trouble. His name is a reference from the videogame Okami, whose protagonist is also known as Shiranui. Kenjiro Hata apparently was playing the game prior to naming Shiranui.
Shiranui, a stray kitten and unnamed then, was found by Hinagiku on her way home after a slight misunderstanding with Ayumu. Hinagiku brought her home because she found Shiranui crying in the middle of the street. Hinagiku together with Ayumu and Hayate took care of Shiranui that night. Hayate was able to gain proper knowledge and money to feed Shiranui through Wataru who he encountered when he was out to buy cat milk.
The following day, a teary eyed Hinagiku had to say goodbye to Shiranui because her mom has a cat allergy. Hayate then decided to take Shiranui and find him a new owner. When Hayate returned to the Sanzenin Mansion, he was immediately welcomed back by Nagi who caught sight of the cat and got startled. At first, Nagi was doubtful as to taking care of the cat but quickly changed her mind when he playfully bit her finger and decided to make the cat a member of the Sanzenin family and gave it the name Shiranui.
This new addition to the mansion would quickly cause a rivalry between the 2 pets currently under Nagi's care. Tama, a white tiger (cat - according to Nagi, Maria, and Isumi), would persistently try to get rid of Shiranui after Nagi told Tama not to enter her room for Shiranui's sake. As a result, Tama was enraged and planned a trap for Shiranui but ended up being the victim of his own trap. As a result, he was temporarily caged in the warehouse and this marked the very beginning of their rivalry.
However the 2 pets do get along when needed. In one instance when Tama got lost in the city they helped each other return back to the mansion. Shiranui helped Tama steal Izumi's phone which he used to ask directions on how to return home. And Tama risked his life to save Shiranui from an incoming truck that was going to hit the cat, but in the end Hayate came just in time to save them both.