Yotsugi appears as a young girl around the age of twelve, with turquoise hair, green eyes, and distinct pointy ears. One of her most recognizable features is her unusual orange hat adorned with googly eyes. She typically wears a frilly orange dress with multiple shades, paired with striped blue and grey stockings and yellow boots.

In Nademonogatari, her hair is cut shorter following her confrontation with God Nadeko. After her original outfit is damaged, she temporarily adopts a new look consisting of a yellow beanie (worn over her signature hat), a green jacket, white shirt, and red pants. Eventually, she reverts to her original attire.

When acting as Tsukihi Araragi’s doll, Yotsugi is often seen dressed in a kimono presumably chosen by Tsukihi to reflect her personal sense of style.

Yotsugi exhibits an outwardly emotionless and expressionless nature, speaking in a consistently flat, monotone voice and rarely changing her neutral facial expression. This subdued behavior is likely linked to her origins as a reanimated corpse and supernatural entity. Despite her seemingly robotic exterior, she gradually explores her sense of identity through distinct verbal tics and gestures. In her early appearances, she repeatedly ends her sentences with the phrase “Boku wa kimegao de sou itta” (“I said that with a posed look”), using the masculine pronoun boku for self-reference. However, she later abandons this catchphrase out of embarrassment, a sign of her developing emotional awareness. When Koyomi Araragi brings it up, she sternly warns him never to mention it again.

Throughout the series, Yotsugi is shown to be heavily influenced by the people around her, a trait often associated with other oddities in the Monogatari universe. She routinely addresses others using the affectionate term “older brother” or “older sister,” regardless of actual age. For example, she calls Koyomi “oni no onii-chan” (“devilish big brother”), referencing his vampiric connection. She also calls Deishuu Kaiki “Kaiki-onii-chan,” often greeting him with a peace sign and the phrase “yay peace peace,” which she continues to use throughout the series much to his irritation.

Yotsugi refers to Yozuru Kagenui, her master, simply as “Onee-chan,” notably omitting the usual honorific. Kagenui later hints that this distinction carries personal significance, though she never elaborates on it.

Despite her outward stoicism, Yotsugi demonstrates the capacity for genuine emotion. She refers to Koyomi as a friend and offers him advice from that perspective rather than as a shikigami. She also expresses reluctance at the idea of harming him, despite her obligation to follow orders. Her behavior at times reflects irritation and boldness, particularly in her interactions with Koyomi and Shinobu Oshino. She mocks Shinobu’s age and frequently engages in playful insults. During her stay at the Araragi household, she develops strong dislike for Tsukihi Araragi, but shows contrasting affection for Nadeko Sengoku, addressing her with nicknames and behaving warmly.

In a lighter vein, Yotsugi also shows a peculiar fondness for well-developed muscles.

Yotsugi Ononoki was originally a human before her death, after which her body became the foundation for her current form. Approximately fifteen years prior to the events of the series, her corpse was reanimated by members of the Occult Research Club as part of an experimental project. Through this process, she was transformed into a tsukumogami-like shikigami, a supernatural being bound to serve a master.

Following her resurrection, Yotsugi was given a choice between two potential masters: Tadatsuru Teori and Yozuru Kagenui. She ultimately chose Kagenui, a decision that led to a lasting rivalry between Kagenui and Teori. Despite her choice, Yotsugi later admits she might have preferred Meme Oshino as her master, had he been presented as an option.

Her resurrection had unintended consequences, as each member of the Occult Research Club involved in reconstructing her body became cursed in the area they worked on. Izuko Gaen, who worked on her head, was cursed with vast knowledge, making her seemingly omniscient. Kagenui and Teori, who worked on her legs, were cursed in such a way that they can no longer touch the ground. Kaiki Deishuu and Meme Oshino, who handled her torso, were also cursed, though the exact nature of their affliction remains unknown.

Yotsugi does not appear to retain any memories from her life before being reanimated and considers the moment of her resurrection as the beginning of her existence. It is also unclear whether her current physical appearance matches how she looked when she was alive.

Yotsugi possesses a unique ability known as Unlimited Rulebook, which allows her to manipulate her body—particularly her fingers—into various forms with explosive effects. One of its primary applications involves reshaping her finger to trigger powerful localized explosions. A short-range variation utilizes her pinky finger to achieve a similar effect on a smaller scale.

In addition to her explosive capabilities, Yotsugi demonstrates enhanced physical strength, often shown lifting people including her master Yozuru Kagenui with just a single finger. She also displays remarkable agility through her ability to make massive leaps that resemble teleportation due to their suddenness and speed.

As a shikigami, her body is modular in nature; her limbs can be detached and reattached at will without causing harm, further emphasizing her artificial and supernatural origin.

• Yozuru Kagenu: Yotsugi’s master and the one she chose to serve as a shikigami. The two share a close but strict relationship, with Kagenui often giving her direct orders and expecting obedience. Despite this, there is a mutual understanding between them, and Kagenui is one of the few people Yotsugi addresses with a form of genuine respect, calling her simply “Onee-chan” without an honorific.

• Koyomi Araragi: Yotsugi frequently interacts with Koyomi throughout the series. While her tone is often blunt or sarcastic, she shows a level of concern for him and refers to him as “oni no onii-chan” (“devilish big brother”), referencing his vampiric nature. Though they often bicker, Yotsugi considers him a friend and has expressed a desire not to harm him, even when ordered to.

• Deishuu Kaiki: Although Kaiki is someone she finds irritating, Yotsugi has a habit of calling him “Kaiki-onii-chan” and greeting him with playful gestures like “yay peace peace,” much to his annoyance. Their relationship is not particularly close, but her attitude toward him remains teasing and oddly familiar.

• Tsukihi Araragi: During her time in the Araragi household, Yotsugi develops a strong dislike for Tsukihi, despite being treated like a doll by her. The tension between them is largely one-sided, with Yotsugi expressing open contempt.

• Nadeko Sengoku: In contrast to her dynamic with Tsukihi, Yotsugi forms a fond attachment to Nadeko. She becomes unusually affectionate, using nicknames and treating Nadeko in a supportive and friendly manner, which stands out given her usual emotional restraint.