In this new era of extensive variability surveys, classical pulsators such as RR Lyrae stars (RRLs) and Classical Cepheids play a crucial role. We focus our attention on the use of RRLs as tracers of the oldest stellar populations of the Galactic halo (Halo) and we compare their properties to those hosted by nearby Milky Way (MW) satellites. These investigations are becoming more and more timely thanks to the large number of Halo field RRLs at our disposal and to the detailed and homogeneous studies that have been conducted, with a significant contribution by our group, on Local Group (LG) dwarf galaxies. In our contribution, we have carefully analyzed the pulsation properties, and we have adopted a statistical approach to compare RRLs in dwarf galaxies and in the Halo. We have concluded that, in a hierarchical cosmological scenario, mainly major mergers (dwarf galaxies with masses larger than Fornax dwarf spheroidal galaxy (dSph) have contributed to the Halo formation and to its chemical enrichment. In a preliminary analysis, we applied this approach to M31 and its satellites. However, to have a sound constraint on the origin of the M31 halo, a more complete observing campaign has to be dedicated to the MW twin galaxy.