Doop was an unknown being that first came into continuity as the X-Force's cameraman, but dropped that team to join the X-Statix after most of the X-Force died. It was later suggested by Captain America that Doop was a weapon made by the US during the Cold War that was essential in the defeat of Russia. In one of the X-Statix's missions, Doop's brain exploded. Many Marvel Universe heroes joined together in order to reassemble the parts of Doop's brain, and, for the most part, were successful. In the end of the hunt, Doop fought with Thor for the last part of his brain, since he was able to live and function on his second brain. Doop won the fight and successfully restored his brain. In that fight, he also demonstrated that he is considered worthy by Thor's mystic hammer Mjolnir enough to lift and wield Mjolnir. Although Doop allegedely died by gunfire along with the other X-Statix in their last mission, another creature that is possibly Doop due to the identical looks was found by Polaris and Havok in Costa Rica. Due to the uncanny resemblance, they named the being Daap.

Powers

Doop has many powers and many that are unrevealed.

While he has demonstrated certain abilities, other powers are attributed to him by various sources which are not clearly backed up by canon evidence. The powers he has actually shown are:

The ability to extend a long, prehensile tongue.

Self-levitation.

Energy blasts from a fragment of his brain.

The power to inflate himself.

Enough strength to knock down Thor.

The power to protect himself and others from psychic or telepathic detection.

Immunity to time manipulation.

The power to safely have body parts removed and put back together.

Storing improbably large objects inside himself.

A small backup brain allows him to survive for a few days without his main brain.

Possibly the ability to duplicate objects, as when he produced a swarm of hammers to attack Thor. (Or he may have simply been storing hammers in himself for some reason.)

Doop's pimple also apparently contained a timeless pocket world in which living beings live out their nightmares.

Although he can only communicate in "Doopspeak", he somehow manages to get his point across to those who have known him for a while.