  We present detailed spectroscopic analysis of the extraordinarily
fast-evolving transient AT2018kzr. The transient's observed lightcurve showed a
rapid decline rate, comparable to the kilonova AT2017gfo. We calculate a
self-consistent sequence of radiative transfer models (using TARDIS) and
determine that the ejecta material is dominated by intermediate-mass elements
(O, Mg and Si), with a photospheric velocity of $\sim$12000-14500km/s. The
early spectra have the unusual combination of being blue but dominated by
strong FeII and FeIII absorption features. We show that this combination is
only possible with a high Fe content (3.5%). This implies a high Fe/(Ni+Co)
ratio. Given the short time from the transient's proposed explosion epoch, the
Fe cannot be $^{56}$Fe resulting from the decay of radioactive $^{56}$Ni
synthesised in the explosion. Instead, we propose that this is stable
$^{54}$Fe, and that the transient is unusually rich in this isotope. We further
identify an additional, high-velocity component of ejecta material at
$\sim$20000-26000km/s, which is mildly asymmetric and detectable through the
CaII NIR triplet. We discuss our findings with reference to a range of
plausible progenitor systems and compare with published theoretical work. We
conclude that AT2018kzr is most likely the result of a merger between an ONe
white dwarf and a neutron star or black hole. As such, it would be the second
plausible candidate with a good spectral sequence for the electromagnetic
counterpart of a compact binary merger, after AT2017gfo.
