Kaoru Niimi is an officer in the Earth Federation Cosmo Force, holding the rank of First Lieutenant, who serves as the Science and Technology Officer and Intelligence Division chief aboard the space battleship Yamato. A woman of considerable intellect from Kyoto, she is distinguished by her sharp analytical mind, her glasses, and a calm, professional demeanor that can sometimes mask deep-seated convictions. Her areas of expertise are diverse; beyond her technical and tactical analysis skills, she also holds a doctorate in psychology, which qualifies her to act as the ship's counselor for the crew.

Before the voyage of the Yamato, Niimi was a junior of Shiro Sanada at the academy and became a trusted assistant in his laboratory, establishing a long-standing professional relationship where she continues to regard him with respect. During this time, she was also romantically involved with Mamoru Kodai, the elder brother of the Yamato pilot Susumu Kodai. However, the couple separated before Mamoru departed on the ill-fated Operation M. Despite the breakup, news of his death hits her extremely hard, revealing a lingering personal grief beneath her composed exterior.

Niimi's greatest internal conflict stems from her strategic beliefs. As the war with Garmillas pushes Earth to the brink of ruin, she becomes a proponent of the Izumo Plan, a pragmatic initiative to evacuate what remains of humanity to a colonizable planet. She is deeply skeptical of the Yamato mission to travel to the distant planet Iscandar, viewing it as a desperate gamble. This skepticism leads her to become a secret supporter of General Serizawa, essentially acting as a mole aboard the Yamato to advance the Izumo Plan. Her allegiance culminates in direct action when she participates in a mutiny led by security chief Shinya Itou. In an attempt to force the ship to return to Earth, she even tries to manipulate navigator Daisuke Shima by feigning romantic interest to gain his support.

However, Niimi is not an ideologue without limits. As the mutiny escalates and Itou becomes increasingly violent, threatening to execute Shima for non-compliance, she has a change of heart. She intervenes to save Shima, tackling Itou and sustaining an injury in the process, an act that demonstrates her principles over her pragmatism. Following the failed uprising, she is confined to the brig, but her technical skills prove indispensable. She is temporarily released to help defuse a powerful Garmillas drill missile embedded in the ship’s wave motion gun, a task she successfully completes with the help of the robot Analyzer. After this act of redemption, Captain Okita tacitly pardons her, and she is welcomed back to her duties as a full member of the crew, having learned a humbling lesson from her misjudgment.

Throughout the journey, Niimi proves to be a highly capable officer. Her notable abilities include code analysis, data processing, and electronic warfare. She is instrumental in creating a computer virus to repel Garmillas boarders and demonstrates her coolness under pressure during the delicate disarmament of the enemy drill missile. In her role as a counselor, she is approachable and provides emotional support to her colleagues, including a notable conversation with Susumu Kodai regarding his late brother. Her character represents the arc of a well-intentioned but misguided extremist who allows her reasoned doubts to lead her astray before she ultimately recommits to her shipmates and their original mission.