Delirium, known to the Greek culture as Mania, is the youngest of the Endless, yet still older than the rest of existence. She is usually quite short, and thin, and looks no older than a fourteen year-old human girl. One of her eyes is blue, and the other is green. Her hair changes style and color constantly, as do her clothes. Her shadow never reflects her shape, and is tangible, like velvet. She is said to smell of sweat, late nights, sour wine, and old leather. Her sigil in the galleries of the other Endless is a multicolored, abstract swirl. Her realm is a chaotic, constantly changing mass of colors 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".) Delirium travels with a "guardian" of sorts, a German Shepherd dog named Barnabas, on indefinite loan from her brother Destruction.
First incarnated as Delight, she transformed into Delirium for reasons even Destiny does not know. She can be seen as a very young Delight in Endless Nights, a collection of stories about the Endless; a statue of her as Delight can be seen in Destiny's garden in Brief Lives, and her official portrait in Destiny's gallery depicts her as Delight, not Delirium, in Season of Mists. She is scatterbrained and easily distracted; 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-colored background, sometimes the background color will match the mood she is in (red for anger, blue and green for calm, etc.). 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 near-white. This effort, as she says later, causes her pain. Her speech as Delight in Endless Nights takes the same form, with somewhat orderly lettering and a faint rainbow background.
The other Endless (particularly Death and Destruction) seem to be fond of Delirium, to varying degrees, and protective of her. She in turn is affectionate towards them, particularly Destruction.