His cage rests on one of the Pipe Organ's keys. He needs to be calmed down in order to get to Stuffus. Mickey must either jump onto a TV or an Anvil Sketch on one of the lower keys to get up onto Stuffus' key, or jump down from one of the upper keys or (similar to Haig) you can just drop an Anvil on his cage. After Stuffus is free, he's very grateful and will help Mickey against the Mad Doctor. However, his "warrior code" says that Mickey will have to pay him one hundred E-Tickets before he can help. Should Mickey choose to take him up on his offer, when he enters the Attic, Stuffus destroys the Beetleworx machine that creates Spinners.
He reappears in the sequel and put in charge of the Mean Street South's paint tower after Mickey and Oswald defeat the Blotworx Dragon, as a replacement for another Gremlin. He is later found trapped in Autotopia. To free him, Mickey must go near an orange car and thin out some toon bushes.
In Roald Dahl's original Gremlins book, where each Gremlin takes his name from the airplane pilot they are paired with, there is no Gremlin Stuffus (indeed, the only named Gremlins are Gremlin Gus and Gremlin Jamface), but there is a pilot called Stuffy with a rather prominent role. It is probable that Stuffy was a nickname for “Stuffus”, and that Gremlin Stuffus was the Gremlin partnered with this pilot.