Izumi is described as a strikingly beautiful young woman with neck-length black hair, which falls slightly to the right side of her face, and distinctive purple eyes. A small beauty mark is located just below the left side of her mouth, adding to her refined appearance.

Her accessories typically include two necklaces, one with a small fish pendant and another adorned with red and green jewels, as well as a bracelet worn on her right wrist. In terms of clothing, she is most often seen in long-sleeved tops paired with shorts, ranging from plaid patterns to plain black designs.

Izumi’s overall presence exudes a calm, dignified, and somewhat motherly aura, befitting her role as the head of the Nase family.

As the head of the Nase family, Izumi presents herself with poise, elegance, and a commanding presence that reflects her position of authority. Outwardly, she is composed and graceful, maintaining a kind and motherly demeanor that inspires respect and trust from those around her. At the same time, Izumi is known for her bluntness, often speaking in a direct and candid manner despite retaining a gentle expression.

Izumi’s personality is shaped heavily by her belief in the responsibilities of Spirit World Warriors. She places duty above personal feelings and expects the same commitment from her younger relatives, Mitsuki and Hiroomi, instilling in them a strict sense of discipline. This uncompromising outlook can make her appear cold or emotionally distant, particularly when she prioritizes the protection of Nagatsuki City over individual attachments.

Nevertheless, Izumi’s actions reveal a deeply protective side, as much of her severity stems from her desire to shield her family and the people under her care. Her willingness to shoulder burdens alone highlights both her strength as a leader and the isolation that comes with it, ultimately creating a contrast between her nurturing exterior and the stern pragmatism that defines her role as the head of the Nase household.

With the absence of their parents, Izumi assumed the role of guardian and mentor to her younger siblings, Mitsuki and Hiroomi, guiding them in the ways of Spirit World Warriors. Acting as both a teacher and authority figure within the household, she instilled in them the discipline and duty expected of the Nase family.

Her methods, however, were often stern and uncompromising. Izumi emphasized the necessity of sacrifice for the sake of fulfilling their responsibilities, particularly impressing this philosophy upon Mitsuki. While her lessons could seem harsh, they profoundly shaped Mitsuki’s personality and outlook, leaving her with a sense of duty that mirrored Izumi’s own.

Glaciate Barriers
As one of the most powerful members of the Nase family, Izumi is exceptionally skilled in the use of Glaciate Barriers. Her mastery allows her not only to contain and repel Youmu but also to affect the consciousness of others. She has demonstrated the ability to submerge a person’s mind into a dormant, limbo-like state, as shown when she incapacitated her secretary and when she suppressed Akihito Kanbara’s human side.

Combat Prowess
Beyond her barrier techniques, Izumi is highly proficient in close combat. She is a master of the naginata and demonstrates the ability to dual-wield weapons with ease, underscoring her versatility as a Spirit World Warrior.

Mitsuki Nase
As Mitsuki’s elder sister and guardian, Izumi played a central role in shaping Mitsuki’s outlook as a Spirit World Warrior. She instilled in her the belief that sacrifices must be made for duty, fostering Mitsuki’s detached and disciplined personality. While Mitsuki respects her, their relationship is complicated by Izumi’s strict and sometimes harsh methods, which often left Mitsuki conflicted between familial love and resentment.

Hiroomi Nase
Hiroomi respects Izumi as the head of the Nase household and once idolized her as a role model. However, their bond becomes strained as Hiroomi grows disillusioned with her rigid worldview. By the end of the anime, he renounces her ideals, vowing not to follow the same path. In the Beyond the Boundary Movie: I’ll Be Here – Future, their relationship comes to a breaking point when Izumi, under the control of a parasite Youmu, becomes an enemy. Despite this conflict, Hiroomi ultimately admits that he should have supported her as family rather than blindly chasing her shadow, allowing them to reconcile before the film’s conclusion.

Akihito Kanbara
Izumi views Akihito as a potential threat due to his Half-Youmu nature. Believing his existence to be dangerous for Nagatsuki City, she manipulates Mirai Kuriyama into attempting to kill him. This places her in direct opposition to Akihito and his friends, reinforcing her reputation as a pragmatic leader who prioritizes duty over personal attachments.

The given name Izumi (泉) translates to “fountain” or “spring.” Her surname Nase (名瀬) is composed of na (名), meaning “name,” “noted,” or “distinguished,” and se (瀬), meaning “swift current” or “rapids.” The surname can be interpreted as “renowned position,” reflecting the high status of the Nase family among Spirit World Warriors.