Michiru Isumi is the second oldest of the four Isumi sisters and serves as the commander of the elite tactical squad A-01, known as Isumi's Valkyries. Before the events of the main story, she joined the Imperial Army despite initial family friction and became a top-scoring cadet at the Hakuryo Base training academy, where she was a student of Sergeant Jinguuji Marimo. A training incident that resulted in the death of one of her squad mates initially caused her to harbor strong resentment toward Marimo, but after confronting her and finding her crying alone in her room, Isumi's attitude shifted and her squad later nicknamed Marimo "the Mad Dog" out of grudging respect.

Isumi is straightforward, pragmatic, and disciplined, with a perfectionist drive that makes her an exemplary soldier and leader. Despite her outward competence and the admiration of her subordinates, she struggles with self-consciousness about how others perceive her and is reluctant to show her softer side, revealing it only to close family and a few trusted individuals like Masaki. She is deeply motivated by a sense of responsibility, having blamed herself for the earlier breakdown of her squadron, and she carries a quiet sense of inferiority compared to her older sister. Her personal philosophy emphasizes protecting the living over mourning the dead, a conviction she instills in her squad through her unwavering motto: "Achieve your mission with all your might. Despair not until your last breath. Make your death count."

In the story, Isumi first appears during the Yokohama Incident while testing the XM3 upgrade, and later becomes an instructor for the protagonist, Takeru Shirogane, helping him recover from psychological trauma by guiding him through therapy. She forms a relationship with him built on blunt honesty and mutual respect, culminating in a tense exchange before the Battle of Sadogashima where she demands that he survive no matter what. During that battle, the 00 Unit Susanoo II malfunctions, and Isumi volunteers to enter it. After attempts to activate the autopilot fail and her squadmate Kashiwagi is killed, Isumi chooses to stay behind and manually trigger the self-destruct sequence, sacrificing herself to annihilate the BETA forces and destroy the Sadogashima hive. In her final moments, she learns that her sister survived the battle and conveys a personal regret to Takeru about not being able to use a certain method with the man she loved. Her squad is later renamed in her honor. Her development from a resentful cadet to a selfless commander who makes the ultimate decision to lay down her life for her mission defines her as a character of duty, sacrifice, and principled leadership.