Functions together with ADGRA2 to enable brain endothelial cells to selectively respond to Wnt7 signals (WNT7A or WNT7B) (PubMed:28289266, PubMed:30026314). Plays a key role in Wnt7-specific responses: required for central nervous system (CNS) angiogenesis and blood-brain barrier regulation (By similarity). Acts as a Wnt7-specific coactivator of canonical Wnt signaling by decoding Wnt ligands: acts by interacting specifically with the disordered linker region of Wnt7, thereby conferring ligand selectivity for Wnt7 (PubMed:30026314). ADGRA2 is then required to deliver RECK-bound Wnt7 to frizzled by assembling a higher-order RECK-ADGRA2-Fzd-LRP5-LRP6 complex (PubMed:30026314). Also acts as a serine protease inhibitor: negatively regulates matrix metalloproteinase-9 (MMP9) by suppressing MMP9 secretion and by direct inhibition of its enzymatic activity (PubMed:18194466, PubMed:9789069). Also inhibits metalloproteinase activity of MMP2, MMP14 (MT1-MMP) and ADAM10 (PubMed:9789069, PubMed:33731436). {ECO:0000250|UniProtKB:Q9Z0J1, ECO:0000269|PubMed:18194466, ECO:0000269|PubMed:28289266, ECO:0000269|PubMed:30026314, ECO:0000269|PubMed:33731436, ECO:0000269|PubMed:9789069}. This is the function of ENSG00000122707 (RECK, reversion inducing cysteine rich protein with kazal motifs).