Final answer:
Lady Macbeth is the character who motivates Macbeth to muster courage, assuring success in their plan to murder King Duncan in the play "Macbeth".
Step-by-step explanation:
The character in "Macbeth" who encourages another to muster courage, promising not to fail in their plans, is Lady Macbeth. In the play, she urges Macbeth to follow through with their plan to murder King Duncan and secure the throne. One of the most compelling lines she utters is: "We fail! But courage to the sticking-place, And we'll not fail." This line encapsulates her role in spurring Macbeth into action, countering his hesitance with her fierce determination.