Milt Krieger is part-historic, part fictional figure. In the play When Muggles Attack, the Knights of Walpurgis are an unofficial group of Magical Law Enforcement personnel tasked with spying on muggle groups that might be a threat to wizards, and Milt is their Watchmaster (squad leader).
In the 1970’s, the real Milt Krieger was involved in weird events. He testified in front of the Wizengamet about dangerous groups of Muggles that knew about wizards. Some believed him, but he was dismissed by most of the magical world.
Milt is pictured here using a muggle communication device.