Arthur Nielsen (nee Weisfelt) was an experienced Warehouse Agent, the father of Scott Mohr, and the surrogate father of Claudia Donovan.
Prior to his career at Warehouse 13, Artie was a cryptographer and worked for the NSA. His employers thought he was selling secrets to the Soviets, unaware that he was actually trading artifacts, assuming they were just rare antiques. He was going to be prosecuted for treason. However, Mrs. Frederic approached him that day, and his life changed forever.
Artie worked for the Warehouse from 1978 until his death in 2028, although his role in ongoing investigations was reduced throughout the last ten years.