Specifically inhibits the cysteine protease staphopain B (SspB) by blocking the active site of the enzyme. Probably required to protect cytoplasmic proteins from being degraded by prematurely activated/folded prostaphopain B. Also involved in growth capacity, viability and bacterial morphology (By similarity). Forms a stable non-covalent complex with prematurely activated/folded SspB. N-terminal residues are not required for inhibitory activity. Inactivated by staphylococcal serine protease (SspA). Belongs to the protease inhibitor I57 (SspC) family.