Natural fibers are increasingly being explored as sustainable alternatives to synthetic reinforcements in polymer composites; however, their inherent hydrophilicity, and poor interfacial compatibility with polymer matrices limit their engineering applications. Bauhinia purpurea L. (BP) fiber possesses promising characteristics for composite reinforcement, yet a systematic optimization of alkali treatment to enhance its performance has not been previously reported. This study hypothesized that controlled NaOH treatment could effectively modify the fiber surface by removing amorphous constituents, thereby improving its characteristics. Accordingly, BP fibers were treated with NaOH concentrations of 5%, 10%, 15%, and 20%, followed by comprehensive physical, chemical, crystallographic, thermal, mechanical, morphological, and surface topography characterization to identify the optimum treatment condition. Among these, the 15% NaOH-treated BP fibers exhibited the most balanced combination of properties, indicating its strong potential as reinforcement for sustainable polymer composites. The optimum 15% NaOH treatment produced fibers with a tensile strength of 498.26 MPa, and tensile modulus of 6.58 GPa. It contained 73.02% cellulose, 7.62% hemicellulose, 19.41% lignin, 0.62% wax, 9.57% moisture, and 2.54% ash, while exhibiting improved thermal stability up to 380.75 °C and reduced water absorption compared with untreated fibers. SEM and AFM analyses revealed a cleaner and rougher fiber surface following alkali treatment, which is expected to enhance mechanical interlocking and promote improved fiber–matrix interactions in composite systems. Overall, the findings demonstrate that controlled alkali treatment (15% concentration), substantially enhances the intrinsic properties of BP fibers and establishes a strong foundation for their future utilization as reinforcement in environmentally sustainable polymer composites.