One idea could be an OC who is a brilliant but unorthodox engineer. They come up with new Jaeger designs that are initially scoffed at but prove crucial in a battle. For example, a Jaeger with the ability to split into multiple smaller units for more flexible combat.
You could create an OC that is part of a support team for the Jaeger pilots. This OC specializes in understanding Kaiju biology. They discover a weakness in the Kaiju that no one else had noticed before. Through their research, they develop a new weapon or strategy that gives the Jaegers a significant advantage in combat.