Designers in complex domains like finance, healthcare, or government are often asked to design for expert workflows they barely understand, while their subject-matter experts are too busy to join every iteration. This case study shows how niche AI “agent personas” became an always-available, domain-aware first pass on ideas—helping a UX professional in fintech to pressure-test concepts, understand upstream/downstream impacts, and arrive at better prototypes faster. In this work, AI serves explicitly as a pre-research layer, not a substitute for real users, formal research, or usability testing, ensuring that human insight remains at the center of the design process.