Thiol protease (PubMed:1586157). Has dipeptidylpeptidase activity (PubMed:1586157). Active against a broad range of dipeptide substrates composed of both polar and hydrophobic amino acids (PubMed:1586157). Proline cannot occupy the P1 position and arginine cannot occupy the P2 position of the substrate (PubMed:1586157). Can act as both an exopeptidase and endopeptidase (PubMed:1586157). Activates serine proteases such as elastase, cathepsin G and granzymes A and B (PubMed:8428921). {ECO:0000269|PubMed:1586157, ECO:0000269|PubMed:8428921}. This is the function of ENSG00000109861 (CTSC, cathepsin C).