Abstract: The traditional dry process for producing regenerated leather relies heavily on synthetic polymers, which results in poor flex resistance, inadequate hygienic properties, and reduced environmental sustainability. To address these issues, a fully biomass-based composite regenerated leather crust was developed. In this approach, chrome-tanned leather shavings (CLS) were used as the primary matrix, natural high-hollowness kapok fibers (KF) were introduced as a long-range bridging phase, and modified starch (MS) served as a green binder. A multi-level structured KF/CLS@MS composite was subsequently prepared via hot-pressing molding. The results demonstrate that gelatinized MS constructs an extensive hydrogen bond network, achieving tight interfacial bonding among the heterogeneous fibers. At a KF content of 2.0% and an MS content of 13.3%, the composite exhibits optimal comprehensive performance: the tensile strength reaches 2.59 MPa, and the flex resistance exceeds 80,000 cycles. Mechanism analysis reveals that the flexible KF creates a “long-range bridging and flexible energy dissipation” effect, significantly enhancing the dynamic fatigue resistance. Furthermore, the unique hollow structure of KF provides the material with excellent air permeability (439 mL/(cm²·h)), water vapor permeability (1118 g/(m²·24 h)), and thermal insulation (thermal conductivity of 0.13 W/(m·K)). This study offers an innovative approach for the high-value utilization of leather solid waste and the development of green, functional regenerated leather products.