Sotto (そっと, Sotto) is a resident of the Tree House. Before the happenings of Event X, Sotto was a young adventurer in search of his father. He would adopt the nickname Mercurio (メルキュリオ, Merukyurio) for a short while to add a sense of mystique to his character. This name, however, became famous, and the "Mercurio Myth" was born. However, the feel of adventure washed away from him after Event X. Seeing the world destroyed and torn apart showed him the true reality that everything faces: destruction.