  We obtained high-resolution infrared spectroscopy and short-cadence
photometry of the 600-800 Myr Praesepe star K2-100 during transits of its
1.67-day planet. This Neptune-size object, discovered by the NASA K2 mission,
is an interloper in the "desert" of planets with similar radii on short period
orbits. Our observations can be used to understand its origin and evolution by
constraining the orbital eccentricity by transit fitting, measuring the
spin-orbit obliquity by the Rossiter-McLaughlin effect, and detecting any
extended, escaping hydrogen-helium envelope with the 10830A line of neutral
helium in the 2s3S triplet state. Transit photometry with 1-min cadence was
obtained by the K2 satellite during Campaign 18 and transit spectra were
obtained with the IRD spectrograph on the Subaru telescope. While the elevated
activity of K2-100 prevented us from detecting the Rossiter-McLaughlin effect,
the new photometry combined with revised stellar parameters allowed us to
constrain the eccentricity to e < 0.15/0.28 with 90%/99% confidence. We modeled
atmospheric escape as an isothermal, spherically symmetric Parker wind, with
photochemistry driven by UV radiation that we estimate by combining the
observed spectrum of the active Sun with calibrations from observations of
K2-100 and similar young stars in the nearby Hyades cluster. Our non-detection
(<5.7mA) of a transit-associated He I line limits mass loss of a
solar-composition atmosphere through a T<10000K wind to <0.3Me/Gyr. Either
K2-100b is an exceptional desert-dwelling planet, or its mass loss is occurring
at a lower rate over a longer interval, consistent with a core
accretion-powered scenario for escape.
