Before you stands a strange reptilian creature covered in gray scales, that sometimes reveal stark white skin beneath them.  He appears to stand only five feet tall, but he seems to always be in a crouch, ready to spring.  When he stands upright, he is about six inches taller.  Slim but well-muscled beneath the scales and ridges, he sports a prehensile tail that is twice as long as he is tall, and can be used for whip-like strikes.  While his face appears man-like, his most human feature is his expressive, brown eyes, despite their internal eyeshine, and the nictitating membranes that flick across them to clean and protect.  Otherwise, he has ridges along his skull, a flatter-than-normal nose, and pebbly lips that seem to always have a marker hanging from them as if it were a cigar, as though a longtime smoker, having quit, just could not quite break the habit.  When he speaks, which is not often, his voice sounds raspy, his silibants seeming to be slightly drawn out.  There is one even stranger thing about Gecko.  On the scales of his left butt cheek, there is a color image of a cherub-like grinning devil that actually is also in the skin beneath.
Dressed in jeans and oversized torso garments (baggy sweatshirts or long coats with deep hoods, often in bright colors because his chameleon-likw ability affect them as well), he almost never wears gloves or shoes, leaving his scale-covered, claw-tipped hands and feet bare no matter what the weather.  His belongings are very minimal, maintaining very a few things other than what he needs to survive his life on the streets.  He carries almost no weapons, other than a handgun and a few knives, usually picking up more temporarily from criminals he beats.
He seems ever alert, quickly assessing threat levels, eyes never staying on any one particular sight, unless it has piqued his interest.  A bit fidgetty, he seems to always want to be in some kind words of motion (cracking knuckles, tapping fingers/toes, licking his pebbly lips), yet is incredibly patient and can be very still when necessary despite this otherwise constant movement.
He tends to be quiet, sometimes appearing and disappearing without warning, and his existence on the streets has been the stuff of urban legend amongst the homeless, the impoverished, and the streetgangs.

But, before there was Gecko, there was Sgt Diego Jimenez, a cigar-smoking, New York born Puerto Rican mestizo, of Spanish-Carib and Filipino ancestry, who was short and slim, with dark hair and eyes, and a tattoo on his left buttock of a cherub-like grinning devil; a devout Catholic, he had always said that his soul belonged to God, so it was up to the Devil to save his ass.
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He was a good soldier, slowly mutated over twenty years after exposure to trace amounts of nerve agent and breathing in aerosolized crude petroleum, aerosolized human and animal excrement, pollution, and torturous experimentation.  During his time in service, he was often plucked from the Military Police company to which he was attached, to other units in far more dangerous missions, including intel gathering/recon in Iraq and Kuwait during Operations Desert Shield and Desert Storm, humanitarian aid in Bosnia-Herzegovina, and more aid as well as hunting of warlords, and a rescue mission in Mogadishu, Somalia.  During this time, he was able to get married and start a family.
When his once olive skin started to become pale white, all his body hair fell out, and his natural aptitudes became full-on powers, he was one of the first recruited into Fire Team Alpha, the US government's premier super team, as he was prepared to retire from military service to spend more time with wife and three children.
Known as Pitstop for his inability to sleep other than as catnaps, he served with Fire Team Alpha fighting terrorists and other enemies of the US, as well as wealthy-industrialist turned mastermind economic terrorist, Timothy J Gump.  When Gump was defeated for good (some say he was killed, some imprisoned), the team disbanded as the veterans who were left retired, and cost over-runs prevented a new Fire Team Alpha.
Then, a year later, Jimenez was taken prisoner by Gump's twin children, Teresa and Terrence.  Every bit as ruthless as their father, the duo tortured Jimenez by experimenting upon him, trying to find ways to instill jis healing and speed upon there own troops, while also trying to find painful ways to kill him slowly.
The remaining members if Fire Team Alpha came out of retirement to find and rescue their Brother at Arms.  After a year of searching, they found him, and, together, Fire Team Alpha destroyed the Gump legacy, and killed the twins.  While the others remained in the spotlight for awhile, Jimenez tried to return to his family and quiet life.
When he discovered his body changing again, he expected to it to be from the torture he had endured.  He saw himself becoming a monster, so he left his family and went into hiding.  Officially, he is dead, the result of suicide by carbomb; a few toes, a pinky, an ear, the tip of his nose, and a piece of skin with a cherub-like grinning devil tattoo, all partially burnt, were all that was found of him.
While he can not bring himself to return as he is to the family he left behind, he has come to grips with his new powers and appearance, and returned to being a hero.  Primarily for the homeless, the indigent, and the impoverished, who helped him to survive on the streets, though he helps anyone in need.  Then, he disappears afterwards like a reptilian apparition.