Mayoi is a fifth-grade elementary school girl. She is 144cm tall and always carries a big backpack, wears a hairband, and sports ponytails. Befitting her age, Mayoi is cheerful, energetic and inquisitive. She makes a lot of spelling mistakes and has a habit of mispronouncing Koyomi's family name. On the other hand, she does not easily trust strangers, and will even fight back against them if they become too nosy.

Ten years ago she was involved in a traffic accident and died because of it. After that she became a ghost and considers herself a "lost snail." She unwillingly causes the people around her to become lost and unable to find their way home. The only people who can see her are people who don't want to go home in the first place. However, after Araragi and Senjougahara help her find the home she was looking for, she changed from an "earthbound spirit" to a "wandering spirit," a fact that she considers to be a bonus, as well as granting her to be free to wander wherever she desires. At the end of Nisemonogatari, she said goodbye to Koyomi, implying that she will not be seeing him again, which will be the basis of the next story, Mayoi Jiangshi. Her family name before her parents got divorced was Tsunade.
In Shinobu Time, she is chased by the darkness together with Koyomi Araragi. It is later revealed by Izuko Gaen that the darkness was targeting Hachikuji because she remained in the town even after she was saved by Koyomi. To stop the darkness, she finally decides that she must pass on. She kisses Koyomi and confesses her love for him before she vanishes.