This study explores functionalized single-walled carbon nanotubes (SWNTs) for in-materio reservoir computing (IMRC) as an efficient information processing hardware. We focused on SWNTs functionalized with potassium carboxylate groups (SWNT–COOK) and examined the impact of adding hydrochloric acid (SWNT–COOK+HCl). Structural analysis showed that functionalization was successfully introduced into the SWNTs. SWNT-COOK formed larger aggregates due to ionic interactions, which were reduced with HCl addition. Electrical characteristics showed that SWNT–COOK exhibited nonlinear properties and higher harmonic generation, which is essential for better RC performance. In benchmark tasks, SWNT–COOK achieved high accuracy across various waveform predictions, outperforming SWNT–COOK+HCl. These results indicate that SWNT–COOK devices have strong potential for developing energy-efficient and high-performance IMRC systems, with performance significantly influenced by the choice of counter ion.