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Twin boundaries (TBs) play an essential role in enhancing the mechanical, electronic and transport properties of polycrystalline materials. However, the mechanisms are not well understood. In particular, we considered that they may play an important role in boron rich boron carbide (B vr BC), which exhibits promising properties such as low density, super hardness, high abrasion resistance, and excellent neutron absorption. Here, we apply first-principles-based simulations to identify the atomic structures of TBs in B vr BC and their roles for the inelastic response to applied stresses. In addition to symmetric TBs in B vr BC, we identified a new type of asymmetric twin that constitutes the phase boundaries between boron rich boron carbide (B 13 C 2 ) and B vr BC (B 14 C). The predicted mechanical response of these asymmetric twins indicates a significant reduction of the ideal shear strength compared to single crystals B 13 C 2 and B 14 C, suggesting that the asymmetric twins facilitate the disintegration of icosahedral clusters under applied stress, which in turn leads to amorphous band formation and brittle failure. These results provide a mechanistic basis towards understating the roles of TBs in B vr BC and related superhard ceramics.