I can't help but wonder. What if she knew that she has slain dozens of people in ways unimaginable?
–Nana, observing the dichotomy between Nyu and Lucy while the former lovingly cradles Piyopiyo
Piyopiyo is a baby bird Nyu finds and takes care of in Chapter 72. The Maple House residents name the bird for the sound it makes, with piyopiyo being the onomatopoeia of a bird's chirping in Japanese.
In the manga
Piyopiyo only appears in the manga, where it appears only for a short time. In Chapter 72, Nyu hears a noise outside and runs to investigate. She finds the baby bird chirping in distress in the snow, having fallen out of its nest somehow, and picks it up off the ground. Nyu asks Kouta if she may be allowed to take care of it, to which he relents but also advises her the bird will need round-the-clock care due to its delicate state. Piyopiyo is left in Nana's care in the next chapter when Yuka and Kouta go to university since she is the only member of the house who doesn't go to school aside from Nyu, who tags along with Yuka and Kouta to their classes.
Piyopiyo doesn't appear again after Chapter 73, so its fate is unknown when the manga ends. It's likely, however, that the residents of Maple House took care of it until it reached adulthood, or simply returned it to its mother after finding its original nest in the garden.
Piyopiyo appears on Chapter 74's cover alongside Nyu, after its last in-series appearance.