The riodinid Rhetus belphegor (Westwood) (n. comb., previously in the genus Nirodia) is a critically endangered butterfly confined to the “campos rupestres”; a high-altitude rocky outcrop vegetation from southeast Brazil. The aim of this study is to unveil its biology and evaluate its systematic position. Based on museum data and public contribution of data (in the context of citizen science), R. belphegor is restricted to the “Espinhaço Mountain Chain”, and occurs exclusively above 1000 m. Adults were found resting upside down on rock walls. Females searched for host plants during the hottest hours of the day, depositing 1–2 eggs on leaves of the herbaceous subshrub Microstachys serrulata (Euphorbiaceae). The non-myrmecophilous larvae developed through six instars and the developmental time from egg to adult was ~50 days. Larvae are covered with abundant setae. Morphology of immature stages and molecular phylogenetic analysis showed that Nirodia is part of Rhetus, justifying the generic change. Our data supports that Nirodia is the only species in its clade associated with high mountains, in contrast to its lowland congeners. The description of the immature biology and clarification on its systematic position are essential steps for the establishment of better and more effective conservation efforts for this magnificent Brazilian butterfly.