  We report the discovery of a buried, active supermassive black hole in SDSS
J085153.64+392611.76, a bulgeless Seyfert 2 (Sy2) galaxy. Keck near-infrared
observations reveal a hidden broad line region, allowing for the rare case
where strong constraints can be placed on both the black hole mass and bulge
component. Using virial mass estimators, we obtain a black hole mass of
log($M_{\rm{BH}}/M_{\odot}$) = $6.78 \pm 0.50$. This is one of the only Sy2 AGN
hosted in a bulgeless galaxy with a virial black hole mass estimate and could
provide important constraints on the formation scenarios of the black hole seed
population. The lack of a bulge component suggests that the SMBH has grown
quiescently, likely caused by secular processes independent of major mergers.
In the absence of a detectable bulge component, we find the $M_{\rm{BH}}$ -
$M_{\rm{stellar}}$ relation to be more reliable than the $M_{\rm{BH}}$ -
$M_{\rm{bulge}}$ relation. In addition, we detect extended narrow Pa$\alpha$
emission that allows us to create a rotation curve where we see
counter-rotating gas within the central kiloparsec (kpc). Possible causes of
this counter-rotation include a galactic bar or disruption of the inner gas by
a recent fly-by of a companion galaxy. This in turn could have triggered
accretion onto the central SMBH in the current AGN phase.
