The Intended is Kurtz's fiancée who stays snug in Belgium (probably eating delicious Belgian waffles and French fries with mayonnaise, hmm, is it lunch time yet?) while Kurtz sails off to gather ivory.
She's beautiful and often connected with imagery of light and heaven:
This fair hair, this pale visage, this pure brow, seemed surrounded by an ashy halo from which the dark eyes looked out at me. Their glance was guileless, profound, confident, and trustful. (3.53).
Check out that halo and the "pure brow": it matches her naïve and idealistic view of Kurtz, who she sees as a kind of saint, whose "goodness shone in every act" (3.70). She's utterly infatuated with Kurtz and believes herself the single most definitive authority on his character: "I am proud to know I understood him better than any one on earth" (3.59). Um, no.
The Intended is essentially a stand-in for every woman, everywhere. (Well, every white, European woman). Her value is measured by her beauty and idealism, and Marlow says that "We must help them to stay in that beautiful world of their own lest ours gets worse" (2.29). In other words, men need women to be beautiful and dumb so there's some bit of goodness in the world. Excuse us while we gag.
But we think it's more complicated than that. (Of course.) Marlow sees women as naïve, idealistic, and gullible—in other words, able to turn blind eyes to the bloody realities and brutalities of imperialism. (Who do you think is wearing all that ivory?) They end up standing in for all Europeans. Like the Intended, white men want to believe in the good and civilizing characteristics of the pilgrims sent into the interior. They want the illusion, and the ivory—not the reality of African slaves worked to death.