Hydrogen (H2) energy has emerged as a principal contender for renewable green energy applications because of the ultra-high energy density and natural abundance. The implementation of this prospective technology necessitates the ultra-high capacity of H2 storage mediums. This work reports the exceptional H2 storage capacities of two-dimensional (2D) carbon allotrope biphenylene (BPL) functionalized by Li, Na, K, and Ca. The combined theoretical approaches including the density functional theory (DFT), ab-initio molecular dynamics (AIMD), maximally localized Wannier functions (MLWFs), and thermodynamic analysis were employed to elucidate the storage efficiencies at operationally practical conditions. The findings reveal that pristine BPL decorated by the selected metals are all inefficient for H2 storage because of the sensitive crystal instability caused by the energetic aggregation of the metallic dopants. On the other hand, point-defected BPL resolves this issue because it adequately magnifies the binding energies with all the decorated metals via the highly ionic bonds. Crucially, these binding energies exceed the cohesive counterparts of the parental metal bulks, consequently stabilizing the crystal integrity. Intriguingly, the Li- and Na-decorated divacancy BPL retain the ultimate H2 storage capacities of 6.76 wt% and 6.66 wt% at the practical temperature and pressure, respectively, surpassing the goal value of 5.50 wt% to be achieved by 2025. Hence, metal-functionalized BPL are conclusively the promising carbon materials for the H2 storage functionality.