<p>Many studies have explored using natural fibers from biomass as reinforcement in thermoplastics. However, a key challenge in developing these composites is achieving effective interfacial adherence between the hydrophilic natural fibers and hydrophobic polymer matrix. Additionally, excessive or insufficient amounts of fiber and compatibilizer can lead to composite failure due to poor dispersion, inadequate interfacial bonding, or fiber agglomeration. In this work, banana fibers at 10, 20, and 30 wt% were incorporated into high-density polyethylene (HDPE) along with 1, 3, 5, and 7 wt% of maleic-anhydride polyethylene (MAPE) as a compatibilizer. The composites were characterized for microstructure, tensile properties, and degree of crystallinity using SEM, FTIR, DSC, and tensile testing. Compared to unfilled HDPE, the addition of banana fiber significantly increased tensile modulus despite limited interfacial bonding. Tensile strength further improved with the addition of 3, 5, and 7 wt% of MAPE to composites containing 30 wt% fibers, suggesting enhanced fiber-matrix adhesion. This was supported by weak FTIR absorption peaks at 1715 and 1740&#xa0;cm^− 1 attributed to esterification between hydroxyl groups and MAPE. Elongation-at-break (EAB), however, decreased with higher fiber content due to the fibers’ rigidity, though MAPE slightly mitigated this effect by improving stress transfer. Differential scanning calorimetry (DSC) indicated that all composites exhibited elevated crystallinity, indicating the formation of additional crystalline regions within the matrix due to fiber inclusion. This effect was further enhanced by MAPE, which improved molecular organization.</p> Graphical abstract <p></p>