Delirium is the youngest of the Endless. She is usually quite short, and thin. Her hair changes style and colour constantly, as do her clothes. Her sigil in the galleries of the other characters is a multicoloured, abstract swirl. Her realm is a chaotic, constantly changing mass of colours and strange objects and shapes, and contains a sundial with the inscription "Tempus Frangit" (time breaks, a Latin pun on the phrase "Tempus Fugit", time flies.)
Until seemingly quite recently, although before the time of the Ancient Greeks at least, Delirium used to be Delight. The change in her character is clearly meant to reflect what Gaiman perceives as change in the basis of the human psyche. Most of the time, she is scatterbrained; she often forgets the thread of her conversations, and comes out with offbeat and seemingly inconsequential observations. Todd Klein, the series' letterer, draws her speech as letters which do not quite match in height or line up neatly against a multi-coloured background, to illustrate this. Very occasionally she is able, with an effort, to become more controlled in thought and speech, at which point her speech is drawn more neatly and the background fades to white.
The other Endless all seem to be fond of Delirium, to varying degrees, and protective of her. She in turn is affectionate towards them, particularly Destruction.
Delirium features in many of the most inventive sequences of the series, particularly in the seventh collection, Brief Lives, in which she and Morpheus attempt to track down Destruction. One of the most striking frames of the whole series features Delirium lying on a hotel bed with a bottle of bubble-blowing liquid, blowing bubbles in a variety of impossible shapes - diamonds, crosses, cats, and small alien beings with umbrellas. Also, in a very important moment in the story, when Destiny imparts upon Dream the information and the means by which he may find Destruction, Delirium manages to collect herself so much that her usual mis-matched appearance disappears, and she becomes a very symmetrical creature, reflecting perhaps that in delirium, delight still exists, no matter how painful.