Kim Arnsbrach studied medicine at John Hopkins Hospital and did her residency in Baltimore. She was eventually recruited by Medicus in order to work on the Med-assist device after developing a cool-headed reputation and earning several honors. During her work on the Med-assist device in the United States, Kim met Mazer Rackham after he participated in a focus group for the device. Mazer and the other New Zealand soldiers had suggested that Kim's voice should be used for the Med-assist, resulting her voice being eventually implemented. Kim later went to New Zealand to work on a localized version of the Medi-assist.
While working on the Kiwi version of the med-assist, Mazer, whom she had been dating, came to talk to her about his brief reassignment to China. Kim was severally disappointed and wanted Mazer to argue his way out of the assignment, which was impossible. Mazer eventually talked about their future and said that he believed it would be bad for Kim and their kids to have an absent father, essentially ending their relationship. Kim left the room and did not come back to talk with Mazer.
During a meeting about the stalling of Med-assist production in China due to the Formic threat, Kim received a phone call from an outdated Med-assist asking for help with an injury. Bingwen contacted Kim looking for help with an intestinal surgery part way through. Kim called in a gastroenterologist to help perform a shadow surgery. Kim helped to persuade Mingzhu to continue the surgery as Dr. Mendelsohn showed her what to do because Mingzhu wanted to quit. Kim soon learned that the patient was Mazer.
Kim was contacted again by Bingwen later when he was at the Dragon's Den. Bingwen told her about the condition of the Dragon's Den before telling her how Mazer was doing after the surgery. Once Bingwen told her Mazer was alive, Kim began to cry but asked Bingwen not to tell Mazer. Bingwen then asked Kim if she could help him set up a system of holopad doctors in the Dragon's Den in order to diagnose patients. Kim agreed, telling Bingwen that she would contact nonprofits.
Sometime thereafter Kime contacted Bingwen using the holopad diagnosis system because Imala Bootstamp and Victor Delgado wanted to see if Bingwen could ellicit the help of the Mobile Operations Police for a mission.
When Mazer came home, he interrupted one of Kim's meetings to see her. They went across the street to a gazebo, where Mazer professed his love to Kim and asked her to marry him. Mazer's ring he gave her was a temporary ring on his holopad, which mimicked his mother's ring. Kim agreed, and Mazer's only condition was that they have a Maori wedding.
After the formation of the International Fleet, Mazer spent one year on Luna. While he was working for the Fleet, Kim was working hard at the Imbrium Memorial Hospital there. During that time, he and Kim tried to have a baby three times, but they all ended in a miscarriage. The week after, Mazer left to work at WAMRED, the Fleet's Weapons and Materials Research Division as the captain of a breach team. While Mazer was away, she frequently sent him mail to inform him on her life.
When Kim found out that Mazer was going back to Luna, she waited for him at the shuttle dock when he arrived. Mazer met Kim outside of the security perimeter. After embracing, they went their favorite noodle shop in the Old Town of Imbrium, which was owned by a Japanese couple they were good friends with after Kim saved their daughter's life. After getting seated and ordering their food, they discussed Mazer's future in the IF. Soon, however, Kim brought up the topic of trying to have a baby again. Mazer was wary at first, for the miscarriages were difficult to get past for the two of them. He didn't want the child to grow up with the Formics still a threat, but Kim persuaded him to think otherwise.
Kim met with Mazer after he was acquitted in court, hoping that Mazer would be discharged. Kim wanted Mazer to remain on Luna, but knew that since Mazer was acquitted that he would be reassigned to some other position. She constantly asked that Mazer request a position on Luna, but Mazer reminded her each time of the unlikelihood of that. Eventually, Mazer parted from Kim, telling her that he believed he would see her later that night between assignments.
The next day Kim was contacted by Mazer after a long night of working at the hospital. Mazer was informing Kim that he had already been reassigned, but fortunately to a safer position than most soldiers. Mazer sang a song to Kim and said he loved her before the call was forcibly ended.
Kim bore Mazer two children, but was divorced by him when he was assigned to a solitary journey at lightspeed to slow his relative aging. Although initially angry with Mazer for divorcing her, she nonetheless still loved him, and spoke to their children with great fondness of their relationship prior to the end of the Second Formic War.
She refused to appear alongside her children and grandchildren in the visual message Hyrum Graff had arranged for the family to send to Mazer, stating that she didn't want Mazer to see her so old. She wrote letters to Mazer, which Hyrum Graff ensured would be forwarded.
In these letters, she was at first angry, but eventually informed Mazer that she had moved on as instructed, remarrying a man with the last name Summers. Once Mazer informed the International Fleet that he would return to Earth to train a new commander for the Third Formic War if they promoted Graff, Graff brought Kim into the Fleet's strategy meetings to help keep the Fleet honest with Mazer. She would laugh when the admirals and generals would try to come up with ways to deceive Mazer, thus upsetting their plans.
Shortly before the Third Invasion, Mazer stated that Kim and his children had died.