Louise Halevy is a central civilian-turned-soldier character whose arc spans the original Mobile Suit Gundam 00 series, its second season, and the film A Wakening of the Trailblazer. She is introduced in the first season as an exchange student from the AEU nation of Spain, studying aeronautical engineering in Japan. She is the girlfriend of Saji Crossroad and initially comes across as outgoing, forward, and accustomed to wealth and privilege. She is not deeply interested in world politics or the Gundam conflict, and she tends to be demanding and jealous in her relationship with Saji, though she clearly cares for him.

Louise's life is shattered during a family wedding ceremony when the Gundam Throne meisters, specifically Nena Trinity, attack the event. The attack kills her entire family and costs Louise her left hand. The wound cannot be regenerated normally due to the lingering effects of red GN particles, and she must later wear a mechanical prosthetic. This trauma fundamentally reshapes her personality. She cuts off her relationship with Saji, believing he would be better off pursuing his dreams without her, and allows a deep, burning hatred for Gundams and for Celestial Being to consume her.

In the second season, she becomes a member of the A-Laws, an autonomous peacekeeping force. As the heir to the Halevy family fortune, she also acts as a major financial sponsor of the organization. She is recruited from a hospital by Regene Regetta and introduced to Ribbons Almark, who uses her as an experimental subject. He provides her with pills containing nanomachines that suppress the cellular damage from GN particle exposure, but these pills are also part of a process meant to artificially evolve her into an Innovator. This treatment grants her the ability to use quantum brainwaves to an extent, but it slowly deteriorates her body. She becomes a mobile suit pilot, serving in units such as the GN-XIII and the customized Ahead Smultron, and eventually receives the large mobile armor Regnant.

Her key relationships are defined by loss and confrontation. Her bond with Saji Crossroad is the emotional anchor of her story, but it becomes deeply strained, as she mistakenly believes he has joined Celestial Being. When they meet again in battle, her hatred and her lingering love for him cause intense internal conflict. She eventually kills Nena Trinity in revenge, but this act does not bring her peace; it pushes her further into instability. During the final battle of the second season, she is overwhelmed by her emotions when she sees the engagement ring she once gave Saji and falls unconscious. She survives thanks to the effects of Setsuna F Seiei's Trans-Am Burst, which purifies the nanomachines in her body, and she reconciles with Saji.

In the movie, set two years later, Louise is in recovery. The cellular damage has been cured, and her lost arm has been restored through regenerative medicine. However, she still suffers from lingering trauma and post-traumatic stress disorder, requiring ongoing hospitalization. Because of the quantum brainwaves she developed during her experimentation, she becomes a target for the ELS. She is saved by Setsuna's intervention and spends the film waiting and praying for Saji's safe return from space. Her development is a story of transformation from a spoiled but lively girl to a grief-stricken, vengeful soldier, and finally to a survivor seeking healing. Her notable abilities, apart from her inherited wealth and political influence, include piloting skill acquired during her time with the A-Laws and a latent capacity for quantum communication due to Ribbons's experiments.