Miyu Miyu is the Nandaba family cat. He is 12 years old, overweight, and has a rather lazy, cranky personality. Haruko uses him to communicate with her superiors. The Galactic Space Police Brotherhood appear to also use him as a video camera, watching Haruko and Naota cautiously.
Miyu Miyu is very lazy when Haruko's superiors aren't using him. He seems to find the interaction between Haruko and Naota amusing. In episode 2, when Takkun tried to play with him, Miyu Miyu ignored him and simply sat down.
Miyu Miyu has gray hair, likely because of old age, and white paws and underbelly. He walks around with a bell on his neck, likely to warn the others of his presence. Despite his apparent obesity, in episode 4 he is shown to be very fast and agile, running in a panic with Naota to the bathroom.
The Galactic Space Police Brotherhood use him as a cell phone of a sort to communicate with Haruko. As shown in episode 5, Haruko may have to move him around to get a good signal. The Brotherhood also uses Miyu Miyu to inflict physical damage on Haruko as shown in episode 2 after Naota's rant.
In episode 2, Miyu Miyu actually speaks to Haruko saying:"Which part is really the truth...Meow?" and "We're talking about you...Meow." This is done during Naota's rant, so it can't be said if it really happened or not. Since Miyu Miyu is seen talking to Haruko afterwards, however, it's likely that it actually happened.