For millennia people have wondered, "Do other Earths exist?" "Are they common?" "Would they show signs of life?". We now have the technical capability to answer these questions. New radial-velocity spectrometers are capable of detecting the reflex motions of stars hosting Earth-mass planets in their habitable zones; the James Webb Space Telescope has the power to probe the atmospheres of rocky exoplanets. Yet the unprecedented precision of these instruments' measurement capabilities is up against a fundamental astrophysically-imposed barrier to achieving these goals: contamination of exoplanetary signals by stellar activity and variability. Further progress is contingent on solving this "variability problem". REVEAL gathers world-leading experts in exoplanetary and stellar physics to tackle this problem in synergy: - We will build on recent advances in magnetohydrodynamic simulations of stellar atmospheres, and data-driven efforts to separate the exoplanet signal from the stellar variability. - We will simulate the "ground truth" of the turbulent physics of entire stellar photospheres resolved at the level of individual convective cells for a broad class of stars. - We will model the emergent spectra of these "virtual" stars and "observe" them using the same data-processing pipelines as stellar radial-velocity and transit-spectroscopy observations. We will continue to observe the Sun and stars hosting small planets found with TESS and PLATO. The stars' own spectra will REVEAL the clues needed to disentangle stellar variability from our measurements of their planets' masses and the fingerprints of molecules in their atmospheres. Our unified efforts will enable the new cutting-edge space observatories and ground-based facilities to realize the full potential of their designs, bringing us closer to the most profound discoveries we could hope to achieve in our lifetimes - the identification of another Earth or even possible signs of life on another planet.