The Mindscape: the dimension which borders on the minds of all sentient creatures. The evil Mindscape entities often seek to enter the minds of sentient beings and drive them mad. Protection comes from the Sleepwalkers, other entities of the Mindscape sworn to prevent the evil beings from entering the minds of innocents. Sleepwalkers are guardians of the Mindscape-- they apprehended beings that invade the sleeping minds of humans. One of these guardians was tricked by his long time foe known as Cobweb into entering the mind of one individual, New York college student Rick Sheridan. The two fought within Sheridan's dreams, but when Cobweb left, the Sleepwalker found himself trapped.
As the Sleepwalker tried to leave Sheridan's mind, he found himself appearing in Sheridan's dreams, or, alternatively, into Earth when Sheridan was asleep. At first, Sheridan believed that the Sleepwalker was just a character in his dreams, until watching the evening news and seeing a report about an attempted robbery. The robbery had been broken up by someone who, according to a police sketch drawn from the description given to them by eyewitnesses, looked exactly like the being in Sheridan's dreams. Sheridan decided to avoid sleep as long as possible, in order to avoid releasing the being again.
When he finally dreamt, Sheridan fought this strangely-dressed, haunting figure. Rick reacted by tearing off Sleepwalker's Imaginator (a badge-like device worn by Sleepwalkers used to teleport around the Mindscape.) This resulted in an odd situation, further trapping the two together: when Rick slept, Sleepwalker could materialize in reality or he could stay in Rick's mind and converse with him via his dreams. It was through these conversations that Sleepwalker was able to dispel Rick's initial fears that he was a malicious entity that wished him harm and work out agreeable terms of cohabitation.
The Sleepwalker learned that Earth was a place full of those who preyed upon the innocent, just like Cobweb and the malevolent entities of the Mindscape. He became a crimefighter, fighting against costumed villains such as 8-Ball, Bookworm, the Chain Gang, and Lullaby. He also teamed up with several of New York's heroes. Once, he joined Spider-Man, the Fantastic Four, and a half-dozen others to defeat the Sinister Six when the villains were backed by the supernatural Gog. Another time, the Sleepwalker became one of Doctor Strange's Secret Defenders against the demonic Roadkill.
It was later revealed that Sleepwalker's race was not one dedicated to protecting Earth but in fact secretly planned to conquer it. When the race tried to invade the world, Sleepwalker decided to rebel. This turned out to be a plot by Sleepwalker's old foe Cobweb to spread mass insanity and thereby gain more power. Cobweb transformed his own minions to appear as the Sleepwalker race and sent them to Earth using Sleepwalker's long lost Imaginator. After reclaiming the Imaginator, Sleepwalker disappeared into the subconscious of his host, Rick.
Recently, Sleepwalker reappeared, forming the League of Losers along with a combination of obscure superheroes Darkhawk, Speedball, Dagger, Araña, X-23, Gravity, Terror, and Mutant 2099. The team was forced to seek refuge in the far future, to defeat the villain Chronok. At this time his link with Rick appeared severed. Sleepwalker remained in the future, as after they "fixed" their past, the heroes realized they were "already there" and therefore unable to return.