Supercritical water gasification (SCWG) of biomass is a promising technology for efficient hydrogen production where the introduction of reactive co-solvents is effective to regulate reaction pathways and enhance gasification performance. In this study, we systematically investigate the effects of co-solvent concentration on SCWG reaction behavior and product characteristics. Corn stover was used as the feedstock with tetrahydrofuran (THF) and N-methyl-2-pyrrolidone (NMP) as co-solvents. The results show that the process exhibits the best synergistic gasification performance at a co-solvent concentration of 10%, where both THF and NMP enhance gas production, while NMP shows a more pronounced effect, achieving a hydrogen volume fraction of 34.45% and a total gas yield increase of 7.98 mmol·g⁻¹. Mechanistic analysis indicates that nitrogen-containing radicals produced from NMP decomposition promote the transformation of aromatic intermediates and suppress condensation reactions, thereby significantly enhancing gasification efficiency, while THF primarily facilitates radical-mediated depolymerization and hydrogen transfer processes, contributing to improved gasification performance. Structural characterization further reveals that the solid residues from NMP system possess higher specific surface area, well-developed mesoporous structures, and abundant nitrogen-doped active sites, and exhibit promising electrochemical energy storage performance. This study elucidates the enhancement of co-solvents in SCWG through regulating radical reaction pathways and solid structural evolution.