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:9789069, PubMed:18194466). Also inhibits metalloproteinase activity of MMP2 and MMP14 (MT1-MMP) (PubMed:9789069). Interacts (via knot repeats) with WNT7A (via disordered linker region); the interaction is direct (PubMed:30026314). Interacts (via knot repeats) with WNT7B (via disordered linker region); the interaction is direct (PubMed:30026314). Interacts with ADGRA2; the interaction is direct (PubMed:30026314). Interacts with MMP9 (PubMed:9789069). Expressed in various tissues and untransformed cells (PubMed:9789069). It is undetectable in tumor-derived cell lines and oncogenically transformed cells (PubMed:9789069). The Kazal-like domains mediate the serine protease inhibitor activity. N-glycosylated. Belongs to the RECK family.