Elium® liquid thermoplastic resin, with room-temperature curing and recyclability, enables large-scale production. However, limited research exists on the fiber–matrix interface, and understanding micro-scale interactions is key to influencing the composite’s macro-scale mechanical properties. This study investigates the interfacial adhesion of glass, carbon, basalt, and aramid fibers-reinforced liquid Elium® thermoplastic matrix composites at micro-, meso-, and macro-scales. Contact angle measurements show 53-56º for glass fibers, indicating superior wettability with the Elium® matrix, while carbon, aramid, and basalt fibers exhibit 58-62º, 73-74º, and 79-86º, respectively. Micro-bond tests demonstrate the highest load-carrying capacity in the interface between glass fibers and the matrix, with glass fibers carrying 11.4% more load than carbon fibers and 25.8% more than basalt fibers. Fiber bundle tests, including transverse and 45° fiber bundle tests, highlight the superior load-carrying performance of glass fibers, with all fiber types showing increased load-carrying capacities in the 45° tests. The micro-scale and meso-scale data obtained from micro-bond and fiber bundle tests corroborated the results of the macro-scale interlaminar shear stress (ILSS) tests, confirming the significant influence of the fiber–matrix interface on the mechanical integrity of the composites. The shear strength at the glass/Elium® interface was 47.54 MPa, which was 8.5% higher than carbon, 20.3% higher than aramid, and 25.9% higher than basalt interfaces. These findings advance our understanding of the mechanical behavior and interfacial adhesion in thermoplastic matrix composites. They underscore the crucial role of the fiber/matrix interface in determining the mechanical properties of composites and offer insights into the compatibility of diverse fiber reinforcements with the innovative Elium® matrix.