Duncan is also a foil to Macbeth, representing all the positive qualities of a king; whereas, Macbeth represents all the negative qualities.
Duncan recognises and rewards loyalty; he is good and gracious.
His honouring of Macbeth after the battle with the title Thane of Cawdor, as well as naming his son, Malcolm, as successor to the throne, seals Duncan’s fate.
His continued trust in Macbeth causes him to place himself in mortal danger by honouring Macbeth with a kingly visit to his home.
We do not see Duncan again after he is welcomed to Dunsinane by Lady Macbeth. His last gracious act is to send a diamond to Lady Macbeth, thanking her for her hospitality.
He epitomises good kingship: he is honest, virtuous, and puts the country's needs before his own.
Duncan's death symbolises the destruction of the political and social order - an order which cannot be restored until another king sits on the throne.
The Scottish rule of succession was not dependant on primogeniture (being the first-born child).
"He was a gentleman (Cawdor) on whom I built / An absolute trust"
"I have begun to plant thee (Macbeth), and will labour / To make thee full of growing"
"We will establish our estate upon / Our eldest, Malcolm, whom we name hereafter, / The Prince of Cumberland"
"There's no art to find the mind's construction in the face"
"This Duncan / Hath borne his faculties so meek" (Macbeth)
"Here lay Duncan / His silver skin laced with his golden blood / And his gashed stabs looked like a breach in nature" (Macbeth)