Rain Mikamura is the sweet and gentle, but also clever and tough young woman who is Domon Kasshu's partner, best friend and mobile suit engineer. She's a brilliant person who helps Domon a lot; though she has to put up with Domon's tough and hotheaded attitude, she still remains loyal and faithful. She was the one who took the picture of Domon and his family which was later ripped in half and used by Domon in his search for Kyoji.
Rain is rather popular with men in the series, due to both her personality and her sensuality. Chibodee Crocket flirts with her openly, George de Sand is specially polite and nice to her (some fans like to think he has a one-sided crush on Rain, which he keeps secret out of respect for both her and Domon), and during her college days she had a brief relationship with engineer Saette Gyuzelle, who would become the Gundam Fighter for Neo Turkey.
Rain studied to become a doctor; however, due to the orders coming from her father, she joined Domon as the engineer of the GF13-017NJ Shining Gundam. She remained with Domon until she was injured while piloting the Shining Gundam in the Guyana highlands. Domon placed her on a Neo-Russian transport and had her go to Neo-Hong Kong while he fought Master Asia and the JDG-00X Devil Gundam.
During the finals she continued to serve as Domon's mechanic until Domon in a fit of rage yelled at her and knocked her to the ground. She then left, and after a meeting with Schwarz Bruder she defected to Neo-Germany, but eventually went back to Domon when he realized that he couldn't go on (physically, emotionally and technically) without her.
Rain VS Allenby
After Schwarz Bruder was injured and revealed to be Kyoji, Rain snuck in to the hospital and heard the truth about what happened when the Devil Gundam was stolen and how her father helped orchestrate a plot along with Ulube Ishikawa to ruin Dr. Kasshu's reptutation. She helped Schwarz escape and get to Lantau island. She then took the JMF-1336R Rising Gundam and fought the Walter Gundam.
Rain Devil Gundam core life unit form
Rain back to Domon's bosom after he resuced her
Rain left for the colony after the Battle Royale because of her father's crimes which she believed were also her crimes since her family honor was forever tarnished and she couldn't face Domon. While on the trip Ulube learned that the ideal pilot for the Devil Gundam was a woman, so he kidnapped Rain and gave her to the Devil Gundam, which then bonded with the colony after feeding off her negative emotions. Domon entered the colony and fought his way to Rain and was able to break her mental and physical barrier by declaring his love for her, Rain broken free from Devil Gundam’s control after she broken the metal layer covered on her body. After he freed her, they finally destroyed the Devil Gundam using the Bakunetsu God Sekiha Love Love Tenkyoken technique.