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 shape-shift her finger.
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).
After being defeated by Shinobu Oshino, she stops saying it. In Koimonogatari series, she started saying “Yaaay” when she first met Kaiki, but he really hated it and she stopped doing so, and started talking normally.
Yotsugi was once a human and was resurrected as a tsukumogami by Yozuru. In Nisemonogatari, Shinobu Oshino mentions that Yozuru “probably is using the name to bind her (Yotsugi).” and she was a supernatural creature “that took on human form.”
She and Kagenui are specialists who specialize in immortal creatures thus they try to kill Tsukihi Araragi who is immortal and is a “monster,” an “Evil Phoenix.” She was defeated by a powered up Shinobu.