Short answers, pulled from the story.
A Gentleman Usher occupied a middle ground between the steward and ordinary servants while overseeing staff who cooked or waited upon the nobleman at meals. These officers ensured the great chamber remained clean under the direction of lesser servants and managed miscellaneous services like the care of chapels and bed-chambers. They were traditionally responsible for swearing in new members of the nobleman's service.
Henry VIII separated them into three distinct classes: Privy Chamber, Daily Waiters, and Quarter Waiters during his reign. Gentlemen Ushers of the Privy Chamber waited at the door of the King's bedchamber and supervised grooms who swept floors and laid straw mats. Under Henry VII, at least one usher attended every day from 8am until nightfall before this separation occurred.
Lieutenant General Ed Davis currently holds the post following Sarah Clarke's appointment on the 17th of November 2017. Since 1971 he has acted as Serjeant-at-Arms of the House of Lords and serves as secretary to the Lord Great Chamberlain. The Gentleman Usher of the Black Rod was established around 1361 as an officer of the Order of the Garter.
Today an establishment of ten Lady and Gentleman Ushers attends royal events and generally comes from retired military officer backgrounds. When on duty they wear service uniforms with brassards displaying the royal cypher or morning dress depending on occasion. These individuals receive modest honorariums for upkeep of their orders of dress and retire at age 70 when they may become Extra Lady or Gentleman Ushers.
Major General James Gordon currently serves as the present Scarlet Rod while Dame DeAnne Julius currently serves as the Blue Rod. Dame Amelia Fawcett currently holds the position of Gentleman Usher of the Purple Rod which appeared in 1918 as usher for the Order of the British Empire. Rear Admiral Christopher Hope Layman currently fills the role of Gentleman Usher of the Green Rod which appeared in 1714 as usher for the Scottish Order of the Thistle.