Final answer:
Lilly Reich assisted Ludwig Mies van der Rohe with the interior design of the Barcelona Pavilion, combining minimalist space use and modern architectural elements.
Step-by-step explanation:
The individual who assisted with the interior design of the Barcelona Pavilion was Lilly Reich. Reich was a close collaborator and partner of Ludwig Mies van der Rohe, a pioneering figure in modern architecture and the director of the Bauhaus school from 1930 to 1933. Mies van der Rohe was responsible for the minimalist design of the reconstruction of the German Pavilion in Barcelona and also famously designed the Barcelona chair, which is considered one of the most recognizable chairs in the world and a tribute to the Bauhaus movement's marriage of design and craftsmanship. While van der Rohe was a leading force in the design of the pavilion, it was Reich who played a significant role in its interior realization.