  We report the chemical abundance pattern of GS\_3073, a galaxy at $z=5.55$
which was previously confirmed to host an overmassive active black hole, by
leveraging the detection of about 40 emission lines, combining JWST/NIRSpec
observations and ground-based (VLT/VIMOS) data. By using rest-frame UV emission
lines, which trace high-density ($\sim 10^5~{\rm cm}^{-3}$) and highly ionized
gas, we derived an abundance ratio of $\rm log(N/O) = 0.46^{+0.12}_{-0.09}$. At
an estimated metallicity of $0.2~Z_{\odot}$, this is the most extreme
nitrogen-rich object found by JWST thus far. In comparison, the relative carbon
abundance derived from the rest-frame UV emission lines is $\rm log(C/O) =
-0.30^{+0.12}_{-0.09}$, which is not significantly higher than those in local
galaxies and stars with similar metallicities. We also detected coronal lines,
including [FeVII]$\lambda 6087$ and potentially [FeXIV]$\lambda 5303$. We
inferred a high Fe abundance of $\rm [Fe/O] \gtrsim 0.1$. Overall, the chemical
abundance pattern of GS\_3073 is compatible with enrichment by super-massive
stars with $M_* \gtrsim 1000~M_\odot$, ejecta from asymptotic giant branch
(AGB) stars, or winds from Wolf-Rayet (WR) stars, although the WR scenario is
less likely. Interestingly, when using optical emission lines which trace lower
density ($\sim 10^3~{\rm cm}^{-3}$) and lower ionization gas, we found a
sub-solar N/O ratio. We interpret the difference in N/O derived from UV lines
and optical lines as evidence for a stratified system, where the inner and
denser region is both more chemically enriched and more ionized. Taking this
luminous, well-studied system as a benchmark, our results suggest that nitrogen
loudness in high-$z$ galaxies is confined to the central, dense, and highly
ionized region of the galaxy, while the bulk of the galaxy evolves more
normally.
