Cory Fields has contributed to Bitcoin Core for over eleven years, with hundreds of commits and close to four thousand review comments. Brink describes his focus as the build system, continuous integration, code security, modularisation and long-term code maintainability. The 0.12.0 release notes, from February 2016, already credit him with build and Gitian work, which gives a sense of how long that thread runs.
None of those areas produce a feature a user can see, and all of them decide whether the software a person downloads is the software the contributors wrote. Reproducible builds, in particular, are what let independent people compile the same source and confirm they get the same binary, so that trusting the release does not mean trusting whoever ran the compiler.
He serves on Brink's grant committee with Mike Schmidt and Niklas Gögge, deciding which Bitcoin protocol engineers the organisation funds.