Gaara is about 12 years old (at the start of the series), and he enjoys killing people that stand in his way (his way of having fun - he hates people and loves only himself). Long ago, when he was around the age of six, his father, the Kazekage, sent assassins to kill off Gaara because he believed Gaara was becoming too dangerous (after killing a man that passed him in the street). But the attempt failed, and later ninja's were sent...who were all killed by Gaara.
Yashamaru says that he is Gaara's mother's brother (that would be an uncle for all you slow people out there). He also tells us, as he is dying, that Gaara's father, the Kazekage (leader of the Village Hidden in the Sand, like Hokage is for the Village Hidden in the Leaves), ordered him to kill Gaara. Yashamaru admits that he could have turned down the mission but chose to do it. Yashamaru had "adopted" Gaara due to his obligation to his sister, but could never get over the fact that Gaara's birth resulted in his sister's death.
Gaara's father was killed by Orochimaru, who took his body and posed as him during the tournament. They find his body after the tournament, and find out about Orochimaru's plot. Gaara has two older siblings, his brother Kankuro and sister Temari, who are with him in the chunin exam.
Gaara's gourd is made of sand and the blood of the victims, which he says gives him power. In battle, he takes out sand and uses it to kill off his enemies by means of the techniques called Desert Coffin and Desert Funeral. The gourd itself is also made out of sand, as is his armor. He has a sleep spell that gives the demon Shukaku the ability to manifest (just check out the dark lines around Gaara's eyes). Gaara doesn't sleep because if he does Shukaku will take control.
Garaa becomes the fifth Kazekage of Sunagakure (village Hidden Among the Sand) after he leaves Konoha, after the chunin exam. Itachi's group kills Gaara by taking the demon from his body, but Naruto and Chiyo end up bringing him back to life.
Editor's Note:
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In Japanese Kanji, Gaara's name is spelled "我愛羅". The kanji on Gaara's forehead, 愛 ("ai"), means "Love", and is the second kanji character is his name. The first kanji in his name means "self", and the third means "carnage". Thus, his name literally means "self-loving carnage". Additionally, before Yashamaru dies, he says that Gaara's name means "demon who cares only for himself". I'd say that matches up pretty good.
Someone stated that in the non-English dub version of the Naruto box set, the subtitles mention that Yashamaru is an "aunt", and I'm told that in both the English and Japanese version of the manga, Yashamaru is indeed female. However, most everyone seems to concur that Yashamaru is a male, so apparently they changed it for the anime, and then Yashamaru was just accidentally labeled as an "aunt". Or perhaps Yashamaru is female in the Japanese version, and they just changed it for the English version, though this is highly unlikely due to the deep male voice and male appearance.
I'm told that typically "maru" is the ending for a boy's name (like "ko" is a typical ending for a girl's name). Apparently, "maru" in the ending of a name is traditionally used for samurai, and parents who named their children with "-maru" wished for them to have bravery, thus the reason it is most often used for boys (chances are that someone wouldn't name a female child 'Yashamaru' because of that). But who knows if that actually means anything in this case. If you have any additional evidence one way or the other, please contact me.
It's no secret that Gaara has siblings, however, someone suggested that his siblings might not actually be his siblings at all, as apparently everyone in the Village Hidden in the Sand refers to each other as brother and sister, which could obviously cause some confusion. There is much evidence, however, that seems to show that the three are in fact siblings. First, in chapters 253 and 254, Kankuro is seen without his makeup and appears to resemble Gaara. Next, in a flashback scene, we see a burning photograph of Temari, Kankuro, Gaara, and the Kazekage. Both Gaara and Kankuro resemble Gaara's known father, the Kazekage, and Temari has the same hair color as Gaara's known mother, who was also seen in a flashback.
Another piece of evidence comes from an episode somewhere in the 74-79 range, where Gaara is having a flashback moment and he talks about how Yashamaru attacked him. It was then that he knew he was to only love himself. At that point in the episode the scene is where you see a younger Temari, Kankuro, and the Kazekage (I am not for sure if Yashamaru is there) all standing together, and it seems as though a camera pans and then you see Gaara, alone. If they are not siblings then why would Gaara think of them, after all they do not learn of their partners until they are a Genin.
Additionally, it's reported in the Shonen Jump manga that they are known as The Sand Siblings.
And finally, arguably the most compelling evidence comes from the episode after Gaara killed three people in the second stage of the Chunnin exams, Kankuro said "Alright, lets head to the tower" and Gaara responded "Just Shut-up", and a few moments later they were arguing and Kankuro grabbed Gaara by the shirt and said "Alright that's enough! Sometimes you've got to listen to what your big brother says!". Gaara's only reply was "Too bad I don't think of you as my siblings at all; if you get in my way, I'll kill you too".
I have had conflicting reports of Gaara's age. Some say 12, some say 14, and many say 13. Thankfully, however, this mystery seems to be over, as in Episode 58 (season 3) Gaara himself makes the statement "These past six years, starting from when I was six...". This would make him 12 years old in Season 3.