  We report on a NICER observation of the Galactic X-ray binary and
stellar-mass black hole candidate, MAXI J1535-571. The source was likely
observed in an "intermediate" or "very high" state, with important
contributions from both an accretion disk and hard X-ray corona. The 2.3-10 keV
spectrum shows clear hallmarks of relativistic disk reflection. Fits with a
suitable model strongly indicate a near-maximal spin parameter of a = cJ/GM^2 =
0.994(2) and a disk that extends close to the innermost stable circular orbit,
r/r_ISCO = 1.08(8) (1-sigma statistical errors). In addition to the
relativistic spectrum from the innermost disk, a relatively narrow Fe K
emission line is also required. The resolution of NICER reveals that the narrow
line may be asymmetric, indicating a specific range of emission radii. Fits
with a relativistic line model suggest an inner radius of r = 144 (+140,-60)
GM/c^2 for the putative second reflection geometry; full reflection models
suggest that radii a few times larger are possible. The origin of the narrow
line is uncertain but a warp likely provides the most physically plausible
explanation. We discuss our results in terms of the potential for NICER to
reveal new features of the inner and intermediate accretion disk around black
holes.
