  A spectroscopic analysis was carried out to clarify the properties of KIC
11145123 -- the first main-sequence star with a determination of
core-to-surface rotation -- based on spectra observed with the High Dispersion
Spectrograph (HDS) of the Subaru telescope. The atmospheric parameters ($T_{\rm
eff} = 7600$ K, $\log g = 4.2$, $\xi = 3.1$ km s$^{-1}$ and $ {\rm [Fe/H]} =
-0.71$ dex), the radial and rotation velocities, and elemental abundances were
obtained by analysing line strengths and fitting line profiles, which were
calculated with a 1D LTE model atmosphere. The main properties of KIC 11145123
are: (1) A low $ {\rm [Fe/H]} = -0.71\pm0.11$ dex and a high radial velocity of
$-135.4 \pm 0.2$ km s$^{-1}$. These are remarkable among late-A stars. Our best
asteroseismic models with this low [Fe/H] have slightly high helium abundance
and low masses of 1.4 M$_\odot$. All of these results strongly suggest that KIC
11145123 is a Population II blue straggler; (2) The projected rotation velocity
confirms the asteroseismically predicted slow rotation of the star; (3)
Comparisons of abundance patterns between KIC 11145123 and Am, Ap, and blue
stragglers show that KIC 11145123 is neither an Am star nor an Ap star, but has
abundances consistent with a blue straggler. We conclude that the remarkably
long 100-d rotation period of this star is a consequence of it being a blue
straggler, but both pathways for the formation of blue stragglers -- merger and
mass loss in a binary system -- pose difficulties for our understanding of the
exceedingly slow rotation. In particular, we show that there is no evidence of
any secondary companion star, and we put stringent limits on the possible mass
of any such purported companion through the phase modulation (PM) technique.
