  Y dwarfs, the coolest known spectral class of brown dwarfs, overlap in mass
and temperature with giant exoplanets, providing unique laboratories for
studying low-temperature atmospheres. However, only a fraction of Y dwarf
candidates have been spectroscopically confirmed. We present Keck/NIRES
near-infrared spectroscopy of the nearby ($d \approx 6-8$ pc) brown dwarf CWISE
J105512.11+544328.3. Although its near-infrared spectrum aligns best with the
Y0 standard in the $J$-band, no standard matches well across the full $YJHK$
wavelength range. The CWISE J105512.11+544328.3 NH$_3$-$H$ = 0.427 $\pm$ 0.0012
and CH$_4$-$J$ = 0.0385 $\pm$ 0.0007 absorption indices and absolute Spitzer
[4.5] magnitude of 15.18 $\pm$ 0.22 are also indicative of an early Y dwarf
rather than a late T dwarf. CWISE J105512.11+544328.3 additionally exhibits the
bluest Spitzer [3.6]$-$[4.5] color among all spectroscopically confirmed Y
dwarfs. Despite this anomalously blue Spitzer color given its low luminosity,
CWISE J105512.11+544328.3 does not show other clear kinematic or spectral
indications of low metallicity. Atmospheric model comparisons yield a log(g)
$\le$ 4.5 and $T_{\rm eff} \approx 500 \pm 150$ K for this source. We classify
CWISE J105512.11+544328.3 as a Y0 (pec) dwarf, adding to the remarkable
diversity of the Y-type population. JWST spectroscopy would be crucial to
understanding the origin of this Y dwarf's unusual preference for low-gravity
models and blue 3-5 $\mu$m color.
