Inez Fressange is a researcher and scientist associated with the Nergal corporation, the private company that built the spaceship Nadesico. Her most significant professional contribution is the design of the Nadesico's propulsion systems, which she reverse-engineered from ancient technological designs that were uncovered on Mars. She is an exceptionally intelligent woman with a calm and analytical demeanor, often appearing suddenly to provide the crew with vital scientific explanations about the phenomena they encounter, making her a key source of exposition within the story. Her personality is driven by a deep scientific curiosity, and she has a notable habit of initiating explanations with the phrase "I'll explain.". To help the crew understand complex situations in an accessible way, she also commissions and hosts the educational segments known as "Naze Nani Nadesico".

In the narrative, Inez is initially encountered by the Nadesico on Mars after the planet has been conquered by the Jovian forces. She was living in an underground shelter with other survivors at the time. When the Nadesico comes to rescue them, a Jovian counterattack tragically kills the other colonists, and Inez is forced to flee aboard the ship, making her the sole survivor of the occupation of Mars. Once aboard, she becomes a resident expert, assisting in analyzing enemy technology and proposing operational strategies. Her true past, however, is the central mystery of her character. It is eventually revealed that Inez is in fact the grown-up version of a young girl named Ai, whom the protagonist Akito Tenkawa had tried to help during the initial attack on Mars years earlier. During that attack, both Akito and Ai were caught in a Boson Jump, a form of instantaneous teleportation. While Akito was sent to Earth, Ai, having never been there, was lost in time and sent back to the era of the ancient Martian civilization. She was given a special artifact and then sent forward in time again, arriving in the Martian desert approximately twenty years before the main events of the series, where she grew into the woman who would become Inez, though her memories of her childhood as Ai were lost.

This hidden history shapes her motivations and her key relationships, particularly with Akito. Her curiosity about him stems not only from his own ability to perform Boson Jumps but also from a deep, subconscious connection to him as the kind person who gave her an orange during the most traumatic moment of her childhood. Later in the story, she becomes instrumental in solving the scientific principles behind Boson Jumping, using her understanding to help the Nadesico execute a critical jump. Throughout the series, she develops from a mysterious, wandering scientist into a character whose personal history is tragically intertwined with the central cast, especially in her final reunion with Akito. As an adult, Inez is a tall, blonde woman with an outstanding figure, typically seen in a white lab coat. Her notable abilities lie not in combat but in her profound scientific knowledge, her skill in mental counseling for the crew, and her unique status as a Jumper, a person capable of initiating a Boson Jump, a power that is central to the series' climax.