The porpoise is a character named "The Porpoise," a friendly and helpful creature that communicates with the other characters in the story through a series of clicks and whistles. It seems Molly understands the Porpoise's "language" as she interacts with it and understands its intentions, but she doesn't learn to speak in the same way.
The Porpoise serves as a symbol of communication and understanding in the story, highlighting the importance of nonverbal communication and empathy.