The Swamp Thing is a humanoid/plant creature elemental, a humanoid mass of vegetable matter who fights to protect his swamp home, the environment in general, and humanity from various supernatural or terrorist threats. His real name is Alec Holland.
Alec Holland, a scientist working in the Louisiana swamps on a secret bio-restorative formula "that can make forests out of deserts". Holland is killed by a bomb planted by agents of the mysterious Mr. E, who wants the formula. Splashed with burning chemicals in the massive fire, Holland runs from the lab and falls into the muck-filled swamp, after which a creature resembling a humanoid plant appears some time later.
Meeting John Constantine
John Constantine first met Swamp Thing in 1985 after being interested with the creature. John later acts as the Swamp Thing's protector, guide, and voice of omen, even teaching the Thing to amplify his powers. Both would have further adventure with each other, such as John introducing the Thing to the Parliament of Trees, Thing using John's body to make love to his wife and father a child named Tefe, and fighting off the Damnation Army from summoning the Anti-Christ. Both carries a dull, but nevertheless fruitful friendship with each other. Constantine even invites Swamp Thing to his 40th birthday and assures the Thing he'll try not to bother him again. (Hellblazer issue 63)
The conclusion of the series Brightest Day revealed that Swamp Thing had become corrupted by the personality of the villain Nekron in the wake of the Blackest Night crossover. In the Brightest Day Aftermath: Search for the Swamp Thing, John Constantine tries to work out what has changed with Swamp Thing, and track him down, with the assistance of Zatanna, Batman and Superman. John managed to discover Alec Holland, who was now a normal human, and tries to get him back to being Swamp Thing, but Alec refuses.
Powers and Abilities
Swamp Thing can inhabit and animate vegetable matter anywhere (including alien plants, even sentient ones) and construct it into a body for himself. As a result, bodily attacks mean little to him. He can easily regrow damaged or severed body parts, and can even transport himself across the globe by leaving his current form, transferring his consciousness to a new form grown from whatever vegetable matter is present in the location he wishes to reach (he even grew himself a form out of John Constantine's meager tobacco supply on one occasion).
Swamp Thing possesses superhuman strength of undefined limits. While Swamp Thing's strength has never been portrayed as prominently as many of his other abilities, he demonstrated sufficient strength to rip large trees out of the ground with ease and trade blows with the likes of Etrigan the Demon.
Swamp Thing can control any form of plant life. He can make it move to his will or accelerate its growth. This control even extends to alien life, as he once cured Superman of an infection caused by exposure to a Kryptonian plant that was driving Superman mad and causing his body to burn out its own power.
After the run of Mark Millar, Swamp Thing had also mastered the elements of fire, earth, water and air, the parliaments of each later killed by The Word, implying that he has retained these abilities and has the power once held by the Parliaments. This has yet to be explained.
The new Swamp Thing (a resurrected Alec Holland) has not only been given the full Elemental powers his predecessor once had, but also a White Lantern Power Ring with full control over its power.