Functions together with ADGRA2 to enable brain endothelial cells to selectively respond to Wnt7 signals (WNT7A or WNT7B) (By similarity). 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. ADGRA2 is then required to deliver RECK-bound Wnt7 to frizzled by assembling a higher-order RECK-ADGRA2-Fzd-LRP5-LRP6 complex. Also acts as a serine protease inhibitor (By similarity). Interacts (via knot repeats) with WNT7A (via disordered linker region); the interaction is direct (By similarity). Interacts (via knot repeats) with WNT7B (via disordered linker region); the interaction is direct (By similarity). Interacts with ADGRA2; the interaction is direct (By similarity). The Kazal-like domains mediate the serine protease inhibitor activity. Localizes to the plasma membrane via its GPI-anchor (PubMed:23329048). Released from the plasma membrane following cleavage of the GPI-anchor by GDPD5/GPE2 (PubMed:23329048). Belongs to the RECK family.