The autistic young mutant known as Dummy became a student of the Xavier Institute before his powers manifested. When they did he became a sentient gas, which was put in a latex suit that grants him a humanoid appearance and functionality. He later takes part in its Special Class, along with other students such as Baslisk, Angel Salvadore and Beak.
Dummy, along with the other students of the special class, follows Xorn as he led them in a field trip to the nearby woods of the Xavier Institute for an overnight camping trip. Soon after they were attacked by the U-Men, humans who harvest mutant body parts and graft it onto themselves. Dummy was struck by a one of the U-Men's scalpel projectiles, and he starts to leak away. He is saved by Beak, who covers the hole in his suit with a condom. Xorn takes care of the majority of the U-Men well away from the Special Class. One soldier on his own threatens the group, but the mutant known as Baslisk stuns him with his powers.
The following day, fellow student Quentin Quire starts a riot over perceived mistreatments. His group take over the main building, while most of the visitors, staff and students are outside. An explosion sends a piece of shrapnel right through Dummy's suit, scattering his gas like form. His suit is cradled by Beak. His final fate is uncertain and though Basilisk claims to later smell him at the Institute's Prizegiving ceremony. Given Basilisk's unusual personality, it was probably just a fart joke.
Dummy exists in a gaseous state; he contains himself in a specially-designed suit.