  We present Hubble Space Telescope (HST) observations of the nearby, massive,
highly rotating hybrid galaxy UGC~12591, along with observations in UV to FIR
bands. HST data in V, I, and H bands is used to disentangle the structural
components. Surface photometry shows a dominance of the bulge over the disc
with H-band B/D ratio of $69\%$. The spectral energy distribution (SED) fitting
reveals an extremely low global star formation rate (SFR) of $\rm\sim0.1-0.2
M_\odot yr^{-1}$, exceptionally low for the galaxy's huge stellar mass of $\rm
1.6\times10^{11}M_\odot$, implying a strong quenching of its SFR with star
formation efficiency of $3-5\%$. For at least the past $\rm 10^{8}$ years, the
galaxy has remained in a quiescent state as a sterile, `red and dead' galaxy.
UGC~12591 hosts a supermassive black hole (SMBH) of $\rm 6.18\times 10^{8}
M_\odot$ which is possibly quiescent at present, i.e. neither we see large
($\rm>1 kpc$) radio jets nor is the SMBH contributing significantly to the
mid-IR SED, ruling out strong radiative feedback of AGN. We obtained a detailed
census of all observable baryons with a total mass of $\rm 6.46\times10^{11}
M_\odot$ within the virial radius, amounting to a baryonic deficiency of
$\sim$$85\%$ relative to the cosmological mean. Only a small fraction of these
baryons resides in a warm/hot circum-galactic X-ray halo, while the majority
are still unobservable. We discussed various astrophysical scenarios to explain
its unusual properties. Our work is a major step forward in understanding the
assembly history of such extremely massive, isolated galaxies.
