Flex Mentallo

This is a weird one.
Doom Patrol

Flex and his "halo"

The original version of Flex Mentallo appeared in Grant Morrison's Doom Patrol. Flex Mentallo was a picked-upon weakling. However, he went through a process that gave him the awesome power of the "muscle mystery". He later investigated a conspiracy involving the Pentagon. He attempted to turn the building into a circle. After he failed, he lost his powers and memories. He was later seen on the Danny the Street. When he regained his memories, he got the help of the Doom Patrol to help them stop the conspiracy. They discovered the Telephone Avatar was a villain in the Pentagon planning to turn the world into a "machine", where nothing is accomplished. TA was using Wally Weaver, a psychic, to further his own goals. It turned out Flex Mentallo is actually a fictional character created by Wally, who also brought him to life via his psychic powers. A dying Wally returned Flex his powers. This allowed Flex to make the Pentagon a circle thus depowering the Avatar. Dorothy used the Candlemaker to destroy the Avatar. Flex left knowing his task was done.
Flex Mentallo Miniseries

Flex fighting Mentallium Man in the original comic
(see below)

Flex also had his own miniseries. While the original issues of the comic had bright colors, the trade paperback recolored the comic so it had more subdued colors and greyness. According to the colorist, the TPB is how the comic was supposed to be and was approved by the creators. The story line moves along two tracks, one focused on Flex, the other on Wally Sage.

Flex learned about the villains Faculty X, who are trying to create social unrest, and
found a clue that pointed towards the Fact (who isn't real). We meet Walley Sage, who is reading a homemade comic about the Fact before finding a phone. Flex revealed Wally had brought him to life before he died and he thought that the Fact may have been brought to life in a similar manner.

The world started to metaphorically crumble around Flex. Despite this, Flex continued to try and find the Fact. Wally started to talk someone on the phone. Flex met an old man, who had gained a crossword that if he finished it he would gain a magic word that would make him a god. Not wanting be a god that looks down on humanity, the old man decided to not use the magic world and gave the crossword to Flex. Wally revealed over the phone he is committing suicide and just wanting to talk to someone before dying. Flex found a slip of photos that had the Fact on them. Flex found an address on the photo and decided to go to it.

A younger version of Wally said Mentallo is in trouble. The "normal" Wally's sense of reality and fiction became blurred because of the overdose of drugs he deliberately consumed. In a bar, Flex met several people that claimed to have met the fictional Legion of Legion superhero team. A faceless man, Mystery Pilgrim, told Flex to find a "teleporter tube", which will lead to the Legion and Faculty X. Wally had a vision of multiple Earths. A lieutenant (from earlier in the story) visited the villain Hoaxer. Wally went more and more crazy while Flex went through a sleazy nightclub. The younger Wally was seen with a Legion member, Lord Limbo.

In a flashback, the Legion's universe was being destroyed by "the Absolute" and they sent to two members with shrinking powers to plant "the seed", which they do. Out of flashback, the lieutenant and Hoaxer arrived at the night club, where they found Flex missing, everyone else dead, and the tube. Flex was teleported to the villain's base. Wally found himself on the moon. He met the younger version of himself. This reminded him of a dream where he met Limbo, who said his imagination is another dimension and the heroes were trapped in it. The young Wally told Wally that Flex is in trouble.

The leader of Faculty X, nicknamed "the Man in the Moon" (due to his moon base and his mask), attempted to kill Mentallo with black mentallium, but Hoaxer and the lieutenant arrived. Wally appeared back on Earth confused. The Hoaxer used his powers of misdirection to save Flex, who used his powers to unmask the Man in the Moon. The Man in the Moon turned out to be a version of Wally. This version was the bridge between Wally's reality and Flex's fantasy. Flex offered to help Wally, which he accepts. The "normal" Wally was now happy and the person he called turned out to be the Fact. Wally found the old man's crossword and finished it. The word turned out to be "Shaman". With the powers of a god, Wally destroyed the barrier between reality and fantasy thus freeing the superheroes.

Powers
In both versions, Flex has the same powers. Flex's powers are ill-defined, but powerful. Mentallo can alter reality when he flexes his muscles. For example, he could turn the Pentagon into a circular shape. When he uses his abilities, the shimmering words "Hero of the Beach", which is referred to as a "Hero Halo", appears over his head.
However in the miniseries, Flex has a weakness (which is not mentioned in the Doom Patrol comics): mentallium. Mentallium is a kryptonite-like substance that had different textures depending on the type. Mentallium has also a different effect depending on the type: black is lethal, pink makes him want him to talk about issues involving sexuality and gender, silver robs him of his sense of humor, ultraviolet can turn him into anyone else (including the memories) and "Lamb and Turkey" has an unknown effect.
References:
http://www.comicvine.com/flex-mentallo/4005-24979/
http://en.wikipedia.org/wiki/Flex_Mentallo
Flex Mentallo miniseries TPB
http://mindlessones.com/2012/04/10/whatever-happened-to-the-mentallium-man-of-tomorrow/
http://tvtropes.org/pmwiki/pmwiki.php/ComicBook/FlexMentallo?from=Main.FlexMentallo