  We present a new analysis of Kepler-21, the brightest (V = 8.5) Kepler system
with a known transiting exoplanet, Kepler-21 b. Kepler-21 b is a radius valley
planet ($R = 1.6\pm 0.2 R_{\oplus}$) with an Earth-like composition
(8.38$\pm$1.62 g/cc), though its mass and radius fall in the regime of possible
"water worlds." We utilize new Keck/HIRES and WIYN/NEID radial velocity (RV)
data in conjunction with Kepler and TESS photometry to perform a detailed study
of activity mitigation between photometry and RVs. We additionally refine the
system parameters, and we utilize Gaia astrometry to place constraints on a
long-term RV trend. Our activity analysis affirms the quality of Kepler
photometry for removing correlated noise from RVs, despite its temporal
distance, though we reveal some cases where TESS may be superior. Using refined
orbital parameters and updated composition curves, we rule out a ``water world"
scenario for Kepler-21 b, and we identify a long period super-Jupiter planetary
candidate, Kepler-21 (c).
