Before Camp Green Lake
Marion Sevillo was arrested for an unknown crime in El Paso, Texas. He was parolled from using or being within a raidious of a fire arm, he then underwent the alias of Mr. Sir at his job at Camp Green Lake.
At Camp Green Lake
While at Camp Green Lake, Mr. Sir was the head counselor. He carried a pistol that he used to kill Yellow Spotted Lizards. Mr. Sir was the first person at Camp Green Lake that Stanley Yelnats IV meets. Mr. Sir has quit smoking by the time Stanley arrives so he is seen eating from a sack of sunflower seeds. Some time passes and Magnet steals his sack of sunflower seeds from his truck. When the boys are passing the sack around, Stanley accidentally spills them all into his hole. Mr. Sir notices his missing sunflower seeds before even getting back to the camp. When he comes back he sees the sack of sunflower seeds poorly hidden in Stanley's hole. Mr. Sir takes Stanley to the Warden, who slashes Mr. Sir's face with her not dried nails. This causes Mr. Sir extreme pain because her nail polish is made of rattlesnake venom. Later Squid asks Mr. Sir "What happened to your face?" Which causes Mr. Sir to become angry and throw Squid to the ground. When Mr. Sir arrives with the water truck, he pours all of Stanley's water onto the ground and hands him an empty canteen. After Stanley steals Mr. Sir's truck and crashes it in a hole after digging, Mr. Sir is left to fix the truck himself. When Stanley is found digging up one of his old holes with Hector and they find Stanley's great-grandfather's treasure stolen by Kissin' Kate Barlow, the Warden, Mr. Sir, and Mr. Pendanski show up to find Stanley and Hector trapped in the hole covered in Yellow Spotted Lizards. Stanley and Hector get out of the hole and the Attorney General of Texas shows up to get Stanley from the camp. However, while there one of the Attorney General's men notices Mr. Sir and recognizes him as Marion Sevillo. Mr. Sir is then arrested for carrying a firearm while on parole.
After Camp Green Lake
Mr. Sir was likely sent to prison for multiple different charges.