Caroline views China as her greatest rival and always tries to best her. However, China views Caroline in a more friendly light and affectionately refers to her as "Caro-chan".
Her father is his sponsor. He was the one who built her Gunpla. She also has a crush on him. She first considers him to be her boyfriend but then moves on to being her fiancé. Seven years later, she and Nils are married.
Seventh World Championship
Caroline is the daughter of the president of the Yajima Corporation which is a major trading company. She views China Kousaka as her greatest rival because she wins regularly in art exhibitions.
After having her art bested by Kousaka's painting of her Gunpla Model (Bearguy III). Caroline decided to avenge her loss in a Gunpla Battle by fighting China in a girls only tournament. She interrupted Sei and China's evaluation of China's Gunpla and taunted Kousaka into accepting a challenge for the tournament.
Neither China nor Caroline knew how to battle, and Caroline had no experience in building. So while China had Reiji and Sei coach her, Caroline used the fact Yajima Corporation was sponsoring the American Researcher and Gunpla World Championship fighter Nils Nielsen to have Nils build her a model and provide her with the needed tactics to best China. He replied by giving her a Knight Gundam and a "foolproof" tactic.
The Knight Gundam's mobility, defense and range of weaponry, along with Nils coaching, were superior to China's Bearguy, since the latter was designed to be a display model rather than battle able. However the use of cotton on the insides of Kousaka's gunpla, an aesthetic choice she made given the backstory she planned for her model, gave her model the ability to reduce the impact of the Knight Gundam's sword, preventing her own model from being destroyed.
Caroline ultimately lost to China after she used the cotton to trap the Knight Gundam and eject it from the Arena. Surprising both Sei Iori, Reiji and Nils Nielsen. Sei would later use the idea to improve his own Gunpla, from which it is assumed he created the RG System (Using Plavsky Particles in lieu of Cotton).
Despite the loss, Caroline soon develops a crush on Nils, as she sneaked into his room to surprise and support him, and outright stated he is her boyfriend. However, Nils seems to accept the role far more reluctantly. This has only escalated with subsequent encounters.
Caroline tries to meet Nils (whom she now considers her fiancé) during Gunpla Eve Festival. When she found Nils, he tries to follow a PPSE truck. The truck got away, but then Caroline asked her butlers to track it. Some time after, they discover an hidden passage and realize they are just below the Tournament Arena. To their shock the hidden passage leads to a huge Plavsky Particle Crystal, but before they could take any further actions they are discovered by Baker and PPSE guards.
Nils and Caroline were bound together within the chamber, making repeated attempts to escape that ended in failure. During the fight with Meijin Kawaguchi in the Finals, however, the Crystal repeatedly reacted to external systems used to control the Meijin, allowing Nils to start deducing what the effects of it could be. The match progressed and the Crystal continued to resonate until the fight's end, where it reacted one final time to create a massive interpretation of the A Baoa Qu fortress that was large enough to break through the stadium's ceiling. Smaller crystals also formed on the arena. Nils and Caroline took this opportunity to escape and warn their allies, but it was too late. Realizing that the only way to stop the disaster was to destroy the large Arista located in the fortress, Caroline along with the other Gunpla Fighters would band together to battle the army of Mocks that were being released from the fortress.
Much later on, Caroline worked with Nils to rediscover the Plavsky Particles, being met with acclaim from the worldwide media.
Thirteen World Championship
She has since made good on her intentions to marry Nils, and has become the director of Yajima Trading, happily flying around the world.