Rainfall-induced instability poses a critical threat to soil slopes. Although fiber reinforcement and vegetation are common stabilization methods, the hydraulic synergy within "root-fiber-soil" systems remains poorly understood. This study addresses the impact of fiber wettability by comparing hydrophilic coir fiber (CF) and hydrophobic polypropylene fiber (PP) combined with ryegrass. Indoor rainfall model tests were conducted to evaluate erosion resistance and hydraulic response. Results indicate a non-linear correlation between fiber content and hydraulic parameters. The optimal dosages for hydraulic regulation were identified as 0.5% for PP and 0.75% for CF. Under heavy rainfall (80 mm/h), the ryegrass-PP system exhibited superior drainage efficiency, reducing peak volumetric water content (VWC) by 10.8% relative to the ryegrass-CF system. Mechanistically, hydrophobic PP facilitates a "drainage synergy" with root channels, accelerating pore water pressure (PWP) dissipation. Conversely, hydrophilic CF induces a "sponge effect", resulting in increased moisture retention. Consequently, a wettability-based design criterion is proposed: hydrophobic fibers are preferred for reducing PWP in high-rainfall regions, whereas hydrophilic fibers are optimal for moisture retention in arid zones.