In recent years, ZrB₂, as a representative material of ultra-high temperature ceramics (UHTCs), has become an important candidate material system for components of new generation aerospace vehicles. However, its practical application is limited by difficulties in material preparation and processing of complex components. This study aims to optimize sintering process of ZrB₂-based UHTCs by introducing HfSi₂ as a sintering aid, specifically addressing the challenge of densification caused by low intrinsic diffusion coefficient of traditional ZrB₂ ceramics. The research focuses on elucidating formation mechanism of core-rim structured borides and their role in enhancing densification of ZrB₂-HfSi₂ ceramics. Dense ZrB₂-HfSi₂ ceramics were successfully fabricated via hot-press sintering at 1600 ℃. The results reveal that softening of HfSi₂ phase during sintering effectively fills interparticle gaps, thereby facilitating low-temperature densification. Furthermore, during the holding stage, interdiffusion of Hf and Zr atoms through a dissolution-reprecipitation mechanism facilitates formation of a core-rim structured ZrB₂/(Zr,Hf)B₂ composite. This core-rim structure consists of ZrB₂ core encased by a (Zr,Hf)B₂ rim, characterized by a fully coherent interface (hexagonal P6/mmm symmetry) with a low lattice mismatch (<5%), ensuring interfacial stability. The ZrB₂-HfSi₂ ceramic exhibits a compressive strength of (1333±83) MPa, a Vickers hardness of (15.86±0.72) GPa, and a fracture toughness of (2.01±0.36) MPa·m^1/2. The ZrB₂-HfSi₂ ceramic demonstrates typical intergranular fracture behavior, with only a limited number of cleavage planes displaying core-rim structural features. These findings provide critical insights into low-temperature sintering of UHTCs and underscore potential of core-rim structures in advancing the preparation of high-performance ceramics.