  We analyse ultra-deep JWST observations of the galaxy JADES-GS-z9-0 at z =
9.4327, and derive detailed stellar and interstellar medium (ISM) properties of
this luminous (MUV=-20.43) high-redshift system. Complementary information from
NIRCam imaging and NIRSpec (both low- and medium-resolution) spectroscopy
reveal a compact system (Re ~110 pc) characterised by a steeply rising star
formation history, which is reflected in the inferred young stellar age (t ~ 3
Myr, light-weighted), high star-formation rate surface density ({\Sigma}SFR ~
72 M yr-1 kpc-2), high ionisation parameter (log(U) ~ -1.5), low metallicity
(12+log(O/H) ~ 7.5), and low carbon-over-oxygen abundance ([C/O] = -0.64).
Leveraging the detection of N iii]1750 we derive nitrogen-over-oxygen abundance
([N/O] ~ 0) higher than the plateau followed by low-redshift galaxies of
similar metallicity, possibly revealing the imprint from (very) massive stars
on the ISM enrichment and favouring a top-heavy Initial Mass Function (IMF)
scenario. Massive stars powering a hard radiation field are also required to
explain the rest-frame UV line ratios, though the presence of the
high-excitation [Ne v]{\lambda}3426 emission line possibly hints at additional
ionization from an AGN. We also report the tentative detection of Ly{\alpha}
emission in the G140M spectrum, shifted by ~450 km/s redward of the systemic
redshift. Combined with a modelling of the Ly{\alpha} spectral break, we rule
out the presence of very high column densities of neutral gas pertaining to
local absorbers, as well as any extended surrounding ionised bubble, suggesting
that JADES-GS-z9-0 has not yet significantly contributed to cosmic
Reionization.
