Nils Yajima, originally known as Nils Nielsen, is a genius physicist and former Gunpla Battle champion who appears as a supporting character in Gundam Build Fighters Try Island Wars. Initially introduced as a thirteen-year-old prodigy in the original Gundam Build Fighters series, he later marries Caroline Yajima, the daughter of his sponsor Yajima Trading, and takes her family name. By the time of Island Wars, he is around twenty years old and serves as the chief technician at Yajima Engineering, where he heads Nielsen Labs on a remote southern island. His primary role in the story is overseeing operational tests for a new battle system that utilizes the New Plavsky Particle, which he refined. This system inadvertently triggers the central crisis of the special, causing a giant Plavsky crystal to form and Gunpla to run wild.

Nils has a calm, analytical personality and treats Gunpla battles not as sources of personal glory but as stepping stones for his research. He shows little outward emotion about victories and does not take pleasure in competition for its own sake. He stopped participating in championship tournaments because he believes his extensive understanding of Plavsky Particles gives him an unfair advantage over other fighters. As a builder and operator, he relies on his deep technical knowledge and natural skill—possibly honed from a background in martial arts—to craft and pilot his signature Gunpla, the Sengoku Astray Gundam, using precise tactical decisions rather than brute force. His key relationships include his wife Caroline Yajima and his professional ties to the Yajima Trading corporation, which supports his work. Over the course of the franchise, Nils develops from a detached young competitor focused solely on particle research into a married engineer who applies his expertise to advancing Gunpla technology, even as he steps away from active fighting. His notable abilities include advanced Gunpla craftsmanship, a tactical mind, and a foundational role in the development of the New Plavsky Particle system that drives the plot of Island Wars.