  Using radial-velocity data from the Habitable-zone Planet Finder, we have
measured the mass of the Neptune-sized planet K2-25b, as well as the obliquity
of its M4.5-dwarf host star in the 600-800MYr Hyades cluster. This is one of
the youngest planetary systems for which both of these quantities have been
measured, and one of the very few M dwarfs with a measured obliquity. Based on
a joint analysis of the radial velocity data, time-series photometry from the
K2 mission, and new transit light curves obtained with diffuser-assisted
photometry, the planet's radius and mass are $3.44\pm 0.12 \mathrm{R_\oplus}$
and $24.5_{-5.2}^{+5.7} \mathrm{M_\oplus}$. These properties are compatible
with a rocky core enshrouded by a thin hydrogen-helium atmosphere (5% by mass).
We measure an orbital eccentricity of $e=0.43 \pm 0.05$. The sky-projected
stellar obliquity is $\lambda=3 \pm 16^{\circ}$, compatible with spin-orbit
alignment, in contrast to other "hot Neptunes" that have been studied around
older stars.
