Finding the emergence of the first metals in the early Universe and identifying their origin are some of the most important goals of modern astrophysics. We present deep spectroscopy of a galaxy at z=12.5, in which we report the detection of the nebular emission line. This represents the most distant detection of a metal transition, and the most distant redshift determination based on emission lines. In addition, we report tentative detections of and and possibly By using the accurate redshift obtained from we can model the drop to reliably measure an absorbing column density of hydrogen of $ $, which is too high for an IGM origin and implies an abundant neutral ISM in or in the CGM around it. We tentatively infer a lower limit for the neutral gas mass of about $10^7 M_ which, compared with the galaxy stellar mass of $ 10^7 M_ implies a gas fraction higher than about 0.2--0.5. By comparing the measured emission lines with model-based diagnostic diagrams, we derive a solar or even super-solar carbon-to-oxygen ratio, tentatively $ (C/O) > -0.21$ dex ($ while a Bayesian modelling of the spectrum indicates $ (C/O) This is higher than the C/O measured in galaxies discovered by at $z=6 9$, and higher than the C/O arising from Type II supernovae enrichment. Asymptotic giant branch stars can hardly contribute to the observed carbon enrichment at these early epochs and low metallicities. Such a high C/O in a galaxy observed 350 Myr after the Big Bang may thus be explained by the yields of extremely metal-poor stars, and may even be the heritage of the first generation of supernovae from Population III progenitors. A robust determination of the total metallicity in this galaxy is essential to constrain these scenarios.