Donalbain
Donalbain is a minor figure whose sole dramatic function is to flee — and to articulate, in a single sharp observation, exactly why fleeing is the right thing to do. His instinct for danger is the clearest thinking in the chaos of Act II.
Appears In
Biography
Donalbain is the younger son of King Duncan. He is present when his father's murder is discovered and speaks only briefly — but his two lines in that scene are among the most practically astute in the play: 'Where we are, there's daggers in men's smiles. The near in blood, the nearer bloody.'
Acting on this assessment, he and Malcolm immediately separate and flee — Malcolm to England, Donalbain to Ireland. Their flight is used by Macbeth's faction to cast suspicion on them, but it saves both their lives.
Donalbain never returns to the action of the play. He has made his calculation and disappeared from the stage, leaving Malcolm to eventually reclaim what was lost.
Motivation
Survival. Donalbain reads the situation at Macbeth's castle with cold accuracy and acts on it immediately.
Personality
Donalbain says almost nothing, but what he says is precise. His observation about daggers hidden in smiles is one of the play's most compressed expressions of its central theme — and he acts on it, which is more than most characters manage.