  PSR J2129-0429 is a "redback" eclipsing millisecond pulsar binary with an
unusually long 15.2 hour orbit. It was discovered by the Green Bank Telescope
in a targeted search of unidentified Fermi gamma-ray sources. The pulsar
companion is optically bright (mean $m_R = 16.6$ mag), allowing us to construct
the longest baseline photometric dataset available for such a system. We
present ten years of archival and new photometry of the companion from LINEAR,
CRTS, PTF, the Palomar 60-inch, and LCOGT. Radial velocity spectroscopy using
the Double-Beam Spectrograph on the Palomar 200-inch indicates that the pulsar
is massive: $1.74\pm0.18 M_\odot$. The G-type pulsar companion has mass
$0.44\pm0.04 M_\odot$, one of the heaviest known redback companions. It is
currently 95\% Roche-lobe filling and only mildly irradiated by the pulsar. We
identify a clear 13.1 mmag yr$^{-1}$ secular decline in the mean magnitude of
the companion as well as smaller-scale variations in the optical lightcurve
shape. This behavior may indicate that the companion is cooling. Binary
evolution calculations indicate that PSR J2129-0429 has an orbital period
almost exactly at the bifurcation period between systems that converge into
tighter orbits as black widows and redbacks and those that diverge into wider
pulsar--white dwarf binaries. Its eventual fate may depend on whether it
undergoes future episodes of mass transfer and increased irradiation.
