It's purple devil with wings, a tail, mauve horns and yellow eyes.
Four years ago, Rinne and his classmates from elementary school were taken on a field trip for some outdoor devil-hunting training, where Devil Balloons were released in the woods. Rinne teamed up with Matsugo, who however in his panic did not help Rinne dispatch the devil before it popped, and the explosion brought about a rift in the boy's friendship that would last for the next four years.
When Matsugo held a Shinigami elementary reunion at Hotel the Beyond, he brought a Devil Balloon as an hint for Rinne to guess who he is.
It expands itself upon detecting a Shinigami, and would blow up if it is not neutralized.