Well, ignore that I said anything in this reply xd.
To unlock the text just talk back to river after you come back at a time that the cat has finished her 'meeting' with river. Of course after you had used the key and seen the scene unfold.
yo la conseguir cuando el chico gato estaba con la diosa del bosque, mientras ambos andaban con lo suyo en la casa del gato apareció la llave de la puerta de river
