Riku is a talking dog who travels with Shizu. He is large, white, and always smiles. Apparently, in the anime, Riku has only spoken to Hermes, as Kino does not believe Riku can speak when told about the discussion Hermes had with him. In the original anime version, Riku also speaks to Shizu, but in the English version, only barks or whimpers to him in these instances. In the novels, Riku speaks to both Kino and Hermes.

(Source: Wikipedia)