  We present HST STIS observations of two extreme NLS1s, IRAS 13224-3809 and 1H
0707-495. The spectra are characterized by very blue continua, broad, strongly
blueshifted high-ionization lines (including \ion{C}{4} and \ion{N}{5}), and
narrow, symmetric intermediate- (including \ion{C}{3}], \ion{Si}{3}],
\ion{Al}{3}) and low-ionization (e.g., \ion{Mg}{2}) lines centered at their
rest wavelengths. The emission-line profiles suggest that the high-ionization
lines are produced in a wind, and the intermediate- and low-ionization lines
are produced in low-velocity gas associated with the accretion disk or base of
the wind. In this paper, we present the analysis of the spectra from these two
objects; in a companion paper we present photoionization analysis and a toy
dynamical model for the wind. The highly asymmetric profile of \ion{C}{4}
suggests that it is dominated by emission from the wind, so we develop a
template for the wind from the \ion{C}{4} line. We model the bright emission
lines in the spectra using a combination of this template, and a narrow,
symmetric line centered at the rest wavelength. We also analyzed a comparison
sample of HST spectra from 14 additional NLS1s, and construct a correlation
matrix of emission line and continuum properties. A number of strong
correlations were observed, including several involving the asymmetry of the
\ion{C}{4} line.
