Desire is the third youngest of the Endless and the twin of Despair. It is a strikingly beautiful figure whose gender is mutable, becoming male, female, both, or neither as the situation warrants. It is often referred to as "sister-brother" by its siblings, particularly Dream. Desire blends in effortlessly with whatever environment it finds itself in. It lives in the heart of a massive flesh-and-blood statue of itself, known as the Threshold. Indeed, further strengthening the connection to hearts, Desire's sigil in the galleries of the other Endless is a heart of cut glass.
Desire is described as being of medium height, smelling faintly of summer peaches. Desire casts two shadows, one black and sharp, the other translucent and wavering. Desire's smiles are brief and sharp. Its skin is "pale as smoke," and its eyes are "tawny and sharp as yellow wine."
Desire is easily the cruelest of the Endless. It seems obsessed with interfering with the affairs of its elder siblings, particularly Dream. The motivation behind this is not clear, but seems to be simply a variation on childish teasing. Desire is not exactly unaware of the consequences of its actions, but considers those consequences ultimately unimportant, a position which angers Morpheus and Death in particular. Desire sometimes acts in concert with Despair and Delirium; the relationship is not clear, however, and Desire is much more distant from its siblings than Despair or Delirium.
A more forgiving interpretation is that Desire reflects, simply, desire, and is as fickle and self-centered as the emotion. As desire is easily the most inflaming of emotions, Desire takes special delight in needling those who think they are beyond emotions altogether. It is, besides Death (who, even then, must be goaded), the only one of the Endless that will point out the faults of Dream, some of which, such as the condemnation of Nada, are legitimate. Even Destruction noted that while Desire can be malicious, it is often right.
A story in Endless Nights, set long before any other Sandman story, explains the origins of the Desire/Dream enmity, and reveals that before this Desire was Dream's favorite sibling. Dream had fallen in love with a mortal, Killalla of the Glow, and had taken her to a gathering of stars (literally the embodiments of various suns). While there, Killalla met and fell in love with the star of her own home world, abandoning Dream. Desire's role in this is never made clear. After introducing his lover to Desire when they first arrive, and leaving the two alone, Killalla asked what Desire had done for Dream to make him so affectionate, but Desire indicated that it hadn't done anything at all. Later, however, when Dream angrily blames Desire for the whole affair, Desire offers no defense, only asking Destruction after Morpheus storms off, "Doesn't he have a sense of humor?" This story, set billions of years before the story-arc of The Sandman, tends to portray the characters in a very different light, showing that even the Endless are not unchanging, with Desire's attitude towards Dream being playful rather than malicious during this early encounter.
The enmity between Dream and Desire continued through the millennia, eventually culminating in Desire raping and fathering a child on Unity Kinkaid, then attempting to have Dream kill Rose Walker, Unity's grandchild, which would have caused the ruthless and relentless Furies to hound Dream until he died for the crime of shedding family blood. When Dream discovered this at the end of The Doll's House, he openly threatened Desire with what would happen should Desire interfere in Dream's life again, and there was an uneasy truce between the two afterwards.