  We present the first spectroscopic transit results from the newly
commissioned Keck Planet Finder on the Keck-I telescope at W. M. Keck
Observatory. We observed a transit of KELT-18 b, an inflated ultra-hot Jupiter
orbiting a hot star ($T_\text{eff} = 6670$ K) with a binary stellar companion.
By modeling the perturbation to the measured cross correlation functions using
the Reloaded Rossiter-McLaughlin technique, we derived a sky projected
obliquity of $\lambda = -94.8 \pm 0.7$ deg ($\psi = 93.8_{-1.8}^{+1.6}$ deg for
isotropic $i_\star$). The data are consistent with an extreme stellar
differential rotation ($\alpha = 0.9$), though a more likely explanation is
moderate center-to-limb variations of the emergent stellar spectrum. We see
additional evidence for the latter from line widths increasing towards the
limb. Using loose constraints on the stellar rotation period from observed
variability in the available TESS photometry, we were able to constrain the
stellar inclination and thus the true 3D stellar obliquity to $\psi =
91.7_{-1.8}^{+2.2}$ deg. KELT-18 b could have obtained its polar orbit through
high-eccentricity migration initiated by Kozai-Lidov oscillations induced by
the binary stellar companion KELT-18 B, as the two likely have a large mutual
inclination as evidenced by Gaia astrometry. KELT-18 b adds another data point
to the growing population of close-in polar planets, particularly around hot
stars.
