They are, as you could've guess are a parasite, however don't make rude assumptions about them, they are here to help! ico is the one being a jerk! always saying "Get out my head" or "you are annoying stop talking" but they are just trying to help, anyway! 

they can also lose their temperament of a friendly guide easily if Ico strays to far from the path they tried to lay out, becoming angry and frustrated. 

no specific requirements for their voice, so do whatever your little heart feels!