Massive ultra-compact galaxies are commonly regarded as nearby relics of the compact quiescent population at z ~ 2--3, and detailed studies have focused so far only on old systems in dense environments. Here we present a spatially resolved analysis of G 79071, an ultra-compact (Re < 2 kpc), massive (M⋆ ~ 1011 M⊙), early-type galaxy (ETG) at z ~ 0.13 residing in a low-mass group environment, based on deep VLT/X-Shooter long-slit spectroscopy. We extracted stellar kinematics out to ~4 Re and constrained stellar population properties and the low-mass end of the stellar initial mass function (IMF), by combining full spectral fitting, full-index fitting, and index fitting, using different stellar population models. G 79071 shows significant rotation, consistent with a fast-rotator-like kinematic structure, and it is dominated by an intermediate-age stellar population (~3--4 Gyr) with a flat age profile and no evidence of a significantly old (≳5 Gyr) component. The central metallicity is supersolar and decreases with radius with a gradient typical of ETGs, while most abundance ratios show flat radial trends and are consistent with typical massive, low-redshift ETGs at a similar velocity dispersion. Moreover, metallicity and [Na/Fe] abundance are significantly enhanced, reflecting a very efficient chemical enrichment process. Our analysis suggests that the IMF remains bottom-heavy out to ~2 Re, implying a mass-excess factor α ≳ 2. Jeans anisotropic models with an NFW halo are consistent with the IMF-based stellar mass normalization and with compactness-corrected (non-homologous) dynamical mass estimators, yielding a modest projected dark-matter fraction within one effective radius, fDM(<Re) = 0.18 ± 0.07. These results show that at least some massive compact galaxies with a bottom-heavy IMF can form and persist at a lower redshift and outside high-density environments, providing new constraints on the diversity of formation pathways for massive galaxies.