Yotsugi was first introduced in Tsukihi Phoenix. She is Yozuru's shikigami. She always addresses herself with the masculine first-person pronoun boku and thinks of Yozuru as her sister. She has an ability called Unlimited Rulebook where she can pretty much do anything, such as, but not limited to, shape-shift her finger and use it as a weapon, or send herself or others flying through the skies.
She seems to know about Koyomi's past as she calls him "Devil Boy" (鬼いちゃん, Oni-chan).
She tends to end the things she says with "I said, with a posed look" (僕はキメ顔でそう言った, Boku wa kime-gao de sou itta).
