  We present Neutron Star Interior Composition Explorer (NICER) observations of
the neutron star low-mass X-ray binary Serpens X-1 during the early mission
phase in 2017. With the high spectral sensitivity and low-energy X-ray passband
of NICER, we are able to detect the Fe L line complex in addition to the
signature broad, asymmetric Fe K line. We confirm the presence of these lines
by comparing the NICER data to archival observations with XMM-Newton/RGS and
NuSTAR. Both features originate close to the innermost stable circular orbit
(ISCO). When modeling the lines with the relativistic line model RELLINE, we
find the Fe L blend requires an inner disk radius of $1.4_{-0.1}^{+0.2}$
$R_{\mathrm{ISCO}}$ and Fe K is at $1.03_{-0.03}^{+0.13}$ $R_{\mathrm{ISCO}}$
(errors quoted at 90%). This corresponds to a position of $17.3_{-1.2}^{+2.5}$
km and $12.7_{-0.4}^{+1.6}$ km for a canonical neutron star mass
($M_{\mathrm{NS}}=1.4\ M_{\odot}$) and dimensionless spin value of $a=0$.
Additionally, we employ a new version of the RELXILL model tailored for neutron
stars and determine that these features arise from a dense disk and supersolar
Fe abundance.
