  We present the results of MAXI monitoring and two NuSTAR observations of the
recently discovered faint X-ray transient MAXI J1848-015. Analysis of the MAXI
light-curve shows that the source underwent a rapid flux increase beginning on
2020 December 20, followed by a rapid decrease in flux after only $\sim5$ days.
NuSTAR observations reveal that the source transitioned from a bright soft
state with unabsorbed, bolometric ($0.1$-$100$ keV) flux $F=6.9 \pm 0.1 \times
10^{-10}\,\mathrm{erg\,cm^{-2}\,s^{-1}}$, to a low hard state with flux $F=2.85
\pm 0.04 \times 10^{-10}\,\mathrm{erg\,cm^{-2}\,s^{-1}}$. Given a distance of
$3.3$ kpc, inferred via association of the source with the GLIMPSE-C01 cluster,
these fluxes correspond to an Eddington fraction of order $10^{-3}$ for an
accreting neutron star of mass $M=1.4M_\odot$, or even lower for a more massive
accretor. However, the source spectra exhibit strong relativistic reflection
features, indicating the presence of an accretion disk which extends close to
the accretor, for which we measure a high spin, $a=0.967\pm0.013$. In addition
to a change in flux and spectral shape, we find evidence for other changes
between the soft and hard states, including moderate disk truncation with the
inner disk radius increasing from $R_\mathrm{in}\approx3\,R_\mathrm{g}$ to
$R_\mathrm{in}\approx8\,R_\mathrm{g}$, narrow Fe emission whose centroid
decreases from $6.8\pm0.1$ keV to $6.3 \pm 0.1$ keV, and an increase in
low-frequency ($10^{-3}$-$10^{-1}$ Hz) variability. Due to the high spin we
conclude that the source is likely to be a black hole rather than a neutron
star, and we discuss physical interpretations of the low apparent luminosity as
well as the narrow Fe emission.
