Daughter of the Japanese scientist Shinzo Mifune, Katsura lost her mother at a young age, which left her and her already embattled father very much alone in the world.

Some years later, while working as an assistant for Dr. Mafune on the control device for the Titanosaurus creature that he discovered and named, Katsura was electrocuted during a test run. Slipping into death as her distraught father attempted to revive her, several mysterious doctors who were conveniently in the area stopped by just in time and saved her life. However, unknown to her grateful father and perhaps to a lesser extent her as well, it came at a price as she began to lose her humanity both physically, by supporting her internal organs with a variety of mechanical mechanisms implanted in her chest, and in terms of her personality.

Years later, an incident involving the destruction of the Japanese submarine Akatsuki that was linked to Titanosaurus lead investigators to the Mafune family. When they stopped by to talk to her family, Katsura lied and said that her father had passed away five years ago before she sent them off and alerted her father, whom was meeting with a man named Tsuda. Both the doctor and her daughter then found out that their benefactors, Tsuda included, were actually the Black Hole aliens and that they were constructing a new Mechagodzilla. After some thought, her father eventually did willingly work for them in building the new mechanical kaiju but Katsura had doubts.

At the same time, one of the investigators, a marine biologist by the name of Akira Ichinose, continued to investigate the details about Dr. Mafune's work and began to confer with Katsura several times to find out more information. Unknowingly at first, they began to develop an attraction to one another while Ichinose attempted to persuade her to come along to help him find out more information and she in turn attempted to warn him off as he was about to travel onboard another Japanese submarine called Akatsuki II.  However, Tsuda got to her first and after stunning her with a laser blast from a gun to her head, persuaded her to unleash Titanosaurus to attack the second sub. Fortunately, the sub managed to fight off the kaiju with the use of its sonar, a feat that shocked both Katsuda and her father upon discovery.

Trying to find out more information, Katsura gave information about Titanosaurus and her father’s work to Ichinose. Learning of the weapon that stunned Titanosaurus, she relayed the information to her father. Trying to persuade her father to rethink his course, Katsura eventually gave in to his wishes. While Titanosaurus attacked mainland Japan, she helped sabotage the supersonic wave weapon, although that nearly cost her life in the process as Japanese soldiers and Interpol agents cornered and then badly wounded her.

This led the Back Hole Aliens to perform the same miracle on Katsura that they did last time she become mortally wounded, but this time the enhanced her capabilities by installing a controller for Mechagodzilla inside. With her humanity almost nearly gone as they transformed her into a cyborg, Katsura worked willingly with the Black Hole Aliens as they unleashed Mechagodzilla and Titanosaurus in an all out savage assault on Tokyo. Even Godzilla could not completely stop the two, although he inflicted heavy damage on the mechanical kaiju while the Japanese military held off Titanosaurus with the repaired supersonic based weapon.

At the same time, a captured Ichinose continuously attempted to reason with her. Even after freeing himself and killing Tsuda, he tried to reach her as she had a gun aimed at him, ready to fire. Fortunately, his friend, Interpol investigator Jiro Murakoshi, arrived and was able to thwart her attempt by shooting her in the shoulder.

Wounded and then seeing her father die before her eyes as the lead commander of the Black Hole Aliens used him as a shield, Katsura started to regain her humanity as she lay in the arms of Ichinose who professed his love to her. Unfortunately, while she lived Mechagodzilla was still very much active and continuing to blast away at Godzilla. Knowing what she had to do, Katsura committed suicide, which finally disabled the mechanical monster and allowed Godzilla to destroy it for the last time.

A saddened Ichinose carried her body up to a cliff and laid her on the ground before Murakoshi, his boss, and several others joined up with him as a victorious Godzilla waded off into the ocean.