  We present the SCExAO direct imaging discovery and characterization of a
low-mass companion to the nearby young A7IV star, HD 91312. SCExAO/CHARIS $JHK$
(1.1-2.4 $\mu m$) spectra and SCExAO/HiCIAO $H$ band imaging identify the
companion over a two year baseline in a highly inclined orbit with a maximum
projected separation of 8 au. The companion, HD 91312 B, induces an
8.8-$\sigma$ astrometric acceleration on the star as seen with the Gaia &
Hipparcos satellites and a long-term radial velocity trend as previously
identified by Borgniet et al. (2019). HD 91312 B's spectrum is consistent with
that of an early-to-mid M dwarf. Hipparcos and Gaia absolute astrometry,
radial-velocity data, and SCExAO/CHARIS astrometry constrain its dynamical mass
to be $0.337^{+0.042}_{-0.044}$M$_\odot$, consistent with - but far more
precise than - masses derived from spectroscopy, and favors a nearly edge-on
orbit with a semi-major axis of $\sim$9.7 au. This work is an example of
precisely characterizing properties of low-mass companions at solar system-like
scales from a combination of direct imaging, astrometry, and radial-velocity
methods.
