The game upon initial release on PC did not contain voice acting for a vast majority of its minor cast, especially one-off characters. A certain supporting / minor cast member also had no sprite whatsoever (which goes for almost all of the minor characters, really).

By the time of the PSP release of Rewrite, that certain character finally had a sprite, and really any side character that needed voice acting was given that. The game is largely voiced at this point. Kotarou's lack of voice acting for a massive majority of the game is intentional and goes unchanged