Recent observations of quantum oscillations (QOs) in transport and thermodynamic parameters at high magnetic fields in a Kondo insulator YbB₁₂ have been a big surprise since it seems to host a Fermi surface, which is a defining character of a metal. In this talk, I will present low-temperature heat-transport measurements to discuss low energy excitations in the ground state of YbB₁₂. At zero field, despite the resistivity ρ_xx being far larger than that of conventional metals, a sizeable T-linear dependent term in the thermal conductivity in the zero-temperature limit, κ⁰_xx/T is clearly resolved, leading to a spectacular violation of the Wiedemann-Franz law: the Lorenz ratio L = κ_xxρ_xx/T is 10⁴-10⁵ times larger than that expected in conventional metals. These data indicate that YbB₁₂ is a charge insulator but a thermal metal, suggesting the presence of itinerant neutral fermions. Remarkably, more insulating crystals with larger activation energies exhibit larger amplitudes of the resistive QOs as well as a larger κ⁰_xx/T, in stark contrast to conventional metals. Moreover, we find that these fermions couple to magnetic field, despite their charge neutrality. Our findings expose novel gapless and highly itinerant, charge-neutral quasi-particles in this unconventional quantum state.