The Guiser or Guisard was the ancient Scotch mummer, who played before Yule; hence our words guise, disguise, guy, etc.
(Thomas). Miser and philanthropist. He amassed an immense fortune in 1720 by speculations in the South Sea Stock, and gave £238,292 to found and endow Guy's Hospital.
Guy Fawkes, or Guido Fawkes, went under the name of John Johnstone, the servant of Mr. Percy.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894