  Ultraviolet (UV) observations of local star-forming galaxies have begun to
establish an empirical baseline for interpreting the rest-UV spectra of
reionization-era galaxies. However, existing high-ionization emission line
measurements at $z>6$ ($\mathrm{W_{C IV, 0}} \gtrsim 20$ {\AA}) are uniformly
stronger than observed locally ($\mathrm{W_{C IV, 0}} \lesssim 2$ {\AA}),
likely due to the relatively high metallicities ($Z/Z_\odot > 0.1$) typically
probed by UV surveys of nearby galaxies. We present new HST/COS spectra of six
nearby ($z<0.01$) extremely metal-poor galaxies (XMPs, $Z/Z_\odot \lesssim
0.1$) targeted to address this limitation and provide constraints on the
highly-uncertain ionizing spectra powered by low-metallicity massive stars. Our
data reveal a range of spectral features, including one of the most prominent
nebular C IV doublets yet observed in local star-forming systems and strong He
II emission. Using all published UV observations of local XMPs to-date, we find
that nebular C IV emission is ubiquitous in very high specific star formation
rate systems at low metallicity, but still find equivalent widths smaller than
those measured in individual lensed systems at $z>6$. Our moderate-resolution
HST/COS data allow us to conduct an analysis of the stellar winds in a local
nebular C IV emitter, which suggests that some of the tension with $z>6$ data
may be due to existing local samples not yet probing sufficiently high
$\mathrm{\alpha/Fe}$ abundance ratios. Our results indicate that C IV emission
can play a crucial role in the JWST and ELT era by acting as an accessible
signpost of very low metallicity ($Z/Z_\odot < 0.1$) massive stars in
assembling reionization-era systems.
