One of the lesser known but oldest characters in the Guardians Of The Galaxy has to be the uniform-wearing walking tree named Groot. He first appeared as an invading extraterrestrial coming to Earth to capture and study humans. This took place in Tales To Astonish #13 all the way back in 1960 and created by Stan Lee and Jack Kirby. Though portrayed as a monster at the time, Groot was merely trying to save his race in his role as the Monarch of Planet X. Sadly though, Groot's plan was defeated by termites.
Groot would pop up three more times in the next fifty years. First, in 1976, he was used by Xemnu along with four other monsters from the Tales To Astonish series against the Incredible Hulk (Annual #5). Its said this was just a duplicate of Groot that was then destroyed by the Hulk. But since Groot has the ability to regrow himself from a sprig… you never know. In 1997 he showed up in a dream that a young Peter Parker was having… like many of us at that age trying to deal with wood that has a mind of its own. And in 2006 he was captured by SHIELD and put into the paranormal containment unit called the Howling Commandos.
So four appearances in fifty years then leads to Annihilation: Conquest where Groot, along with his good friend Rocket Raccoon joins the Guardians Of The Galaxy and basically became the teams "Kenny". Groot sacrificed himself to allow the team to escape. But a sprig offshoot was saved and Groot regrew and was able to assist in the assault on the Babel Spire. When the initial plan of blowing up the spire failed, Groot used his ability to absorb wood as food and grew massive in size. Mantis then turned his sap flammable and the giant tree was then set aflame destroying the spire. Luckily Rocket Raccoon had saved a clipping of his friend (kind of creepy) to which Groot regrew from.
In his adventures with the Guardians, Groot has faced the Badoon, Blastaar, forces from the Negative Zone, The Shi'Ar, The Magus and the Cancerverse. He battled a monster made from Badoon corpses and helped Black Bolt's crazy brother Maximus the Mad build a weapon to fight a giant squid monster. Helping design and build the weapon showed that Groot has an advanced understanding of quasi-dimensional super-positional engineering. The weapon created a Feedback Loop which would uncreate the creature. After that he also encountered the Universal Church of Truth and some happy little guy named Thanos.
Groot appears to be the last of his race and it's unknown if his abilities are specific to him or just natural to his species. He can absorb wood as food, rebuild himself and even grow larger and stronger, but the bigger he gets the less he can move. Though wood, he seems to be fire resistant and he has the ability to control trees, using them to attack his enemies. As part of the royal lineage, he has had the finest education which explains his understanding of engineering. Though he only says, "I am Groot!", Maximus claims that Groot says many things but the hardened nature of his larynx makes it impossible for most to hear the subtle nuances of his speech. Then again, Maximus is insane.
Groot will be joining Rocket Raccoon, Gamora and Starlord in the upcoming James Gunn helmed motion picutre. Kevin Feige, the driving force behind the Marvel Studios cinema world, has said that Groot and Rocket will steal the movie, though at this time no actors (voice actors I would assume) have been cast.
I find it fascinating that someone, most likely Dan Abnett and Andy Lanning, decided to take a monster from 1960 and add it to their galactic superhero team. Perhaps they were influenced by his inclusion in the Nick Fury's Howling Commandos series by Keith Giffen and Eduardo Francisco. Either way, we end up with a hero that predates the Fantastic Four, that can make the ultimate sacrifice multiple times and can even put a smile on Tolkien fans' faces.