A genetic creation produced by the Doress foundation, Sonny-Steffan Nottsuo is a nocturnal marsupial. The creature looks fairly normal, with the exception of its purple hued eyes and giant tail, but displays several unusual traits such as concealed spines in its legs and the ability to glide with its tail against a heavy wind current.

What the Doress company had in mind for the marsupial is unknown, though, as it doesn't appear to serve any militaristic purpose. Regardless, Nottsuo eventually fled from the firm that spawned it and ended up following the young girl Sumire around as she escaped from the organization's clutches with the help of Hasizawa Ikuroo. After the destruction of the Doress laboratories, Nottsuo remained a loyal pet to the young girl as she attempted to live a normal life.