The lowest metallicity stars in the Milky Way Halo are the fossil records of the earliest star-forming environments in the universe. Their chemical abundance patterns help us understand primordial nucleosynthesis, the mass function of the first stars, and the pathways that led to the chemical complexity we observe today. However, there is still debate about when (and for how long) the universe transitioned from metal-free to the first chemical enrichment episodes that triggered low-mass star formation. Furthermore, empirical evidence shows that all the chemically pristine stars observed to date have carbon in their atmospheres at varying levels, suggesting that this element is a key contributor to the chemical evolution of the universe at high redshifts. In this talk, I will review some of the theoretical work on the nature of the first stars and present the discovery of an ultra metal-poor star in the halo of the Galaxy with one of the most pristine chemical compositions ever found. I'll also talk about a serendipitous discovery of a star selected from the same sample that was thought to be ultra-metal-poor, but GHOST data revealed a surprising chemical composition.