Head of the Brotherhood of Dada and enemy of the Doom Patrol, Mister Nobody first appeared in Doom Patrol #26 in a story entitled "Nowhere Man". In that tale he revealed that he had once been an ordinary and rather forgettable human being named Mr. Morden, a member of the original Brotherhood of Evil (only no one remembers him). After the Brotherhood's defeat by the Doom Patrol, Morden submitted to being experimented on by a former Nazi scientist hiding in the jungles of South America in order to gain superpowers and take revenge on the Patrol.
The experiments snapped his mind and transformed him into a bizarre figure more like an artist's abstract than a living being: his body became a mere sliver of inky black, his eyes seemed to float on either side of his head, somehow it was never quite possible to see him properly. "Herr Niemand," the shocked scientist whispered as Morden stepped out of the wreckage of his experimental chamber -- Mister Nobody. Morden killed everyone in the complex and set off alone into the jungle.
He led the first version of the Brotherhood of Dada in their attempt to take over the world (beginning with Paris) by "feeding" it to a painting that draws things into itself. Though the Brotherhood was trapped in the painting afterward, Mister Nobody was able to free himself and form a second group in Issue # 49, "Death In Venice". He acquired the bicycle that chemist Albert Hoffman was riding when he took the first LSD trip ever and hooked it up to the engine of a spectacularly-painted schoolbus so that reality warped around it wherever it went.
Mister Nobody's plan this time was to run for President of the United States -- a story arc probably inspired by an old Betty Boop cartoon in which Betty ran for President against an opponent named "Nobody" who bears a marked resemblance to the Doom Patrol character. The Patrol was flummoxed; though they felt like it should be their duty to stop him, Mister Nobody was, in his admittedly strange way, making a lot of sense. As support for his campaign built, officials at the Pentagon persuaded one of the members of the Brotherhood to betray Nobody and dispatched a super-powered and obviously insane agent named John Dandy to assassinate him.
When Nobody unveiled the painting that ate Paris at a rally and invited the crowd to break down the barriers between it and this world, soldiers descended on the scene and fired tear gas. John Dandy turned Mister Nobody human again and impaled him on a pole as the soldiers burned the painting.
Writer Grant Morrison used the character to convey a great deal about Dada and Situationist philosophy. Though ostensibly a villain, the book's heroes were often forced to consider the validity of the character's absurdist challenges to the status quo. Plus, he was a hell of a lot of fun.
Sources: my own collection, and http://www.rpi.edu/~bulloj/Doom_Patrol/index.html