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We present JWST/NIRSpec Integral Field Spectrograph rest-frame optical dataof the compact $z=5.55$ galaxy GS_3073. Its prominent broad components inseveral hydrogen and helium lines (while absent in the forbidden lines), andthe detection of a large equivalent width of He II$\lambda4686$, EW(He II) ~20A, unambiguously identify it as an active galactic nucleus (AGN). We measurea gas-phase metallicity of $Z_{\rm gas}/Z_\odot\sim0.21^{+0.08}_{-0.04}$, lowerthan what has been inferred for both more luminous AGN at similar redshift andlower redshift AGN. We empirically show that classical emission line ratiodiagnostic diagrams cannot be used to distinguish between the primaryionisation source (AGN or star formation) for such low-metallicity systems,whereas different diagnostic diagrams involving He II$\lambda4686$ prove veryuseful, independent of metallicity. We measure the central black hole mass tobe $\log(M_{\rm BH}/M_\odot)\sim8.2\pm0.4$ based on the luminosity and width ofthe broad line region of the H$\alpha$ emission. While this places GS_3073 atthe lower end of known high-redshift black hole masses, it still appears to beover-massive compared to its host galaxy properties. We detect an outflow withprojected velocity $\gtrsim700$ km/s and infer an ionised gas mass outflow rateof about $100\ M_\odot/$yr, suggesting that GS_3073 is able to enrich theintergalactic medium with metals one billion years after the Big Bang.

GA-NIFS: A massive black hole in a low-metallicity AGN at $z\sim5.55$ revealed by JWST/NIRSpec IFS