(This is for the fanfiction "Crossover Together". Info may change from canon information)
Gaara has fair skin, green eyes, and short auburn hair; for most of his life his hair is spiky, but by adulthood it is combed and parted to the right. He lacks distinctive pupils or eyebrows, the latter of which others sometimes make fun of. He's had tanuki-like black rings around his eyes his whole life, which Gamabunta attributes to insomnia. As a child, he carved the kanji for "love" (愛, ai) on the left side of his forehead, which his hair is parted in order for it to be kept visible.
Gaara wears a black body suit with an open neck, t-shirt-like sleeves, and almost full-length leggings. With this, he wears a white cloth over his right shoulder and the left side of his hips. He has a wide leather band from his left shoulder to his right hip with which he carries his sand gourd and around which he wraps his forehead protector. In early chapters of the manga, he also has several earrings.
Gaara was a man out for himself: only getting involved in situations if there was a benefit from it for him to gain. If he doesn't see anything to gain for helping someone out, then he wouldn't bother with them in anyway. He is a very violent boy, and often responds with violence and death threats. It was revealed later on that this was so people would leave him alone in fear Shukaku would harm them.
Much of his isolation is blamed on Shukaku, as he had to run away from his own tribe because of him. Overtime, he had developed insomnia due to threats from Shukaku that he will take full control if he ever were to fall into a deep sleep. It was also because of Shukaku that he will take any method or chance to get rid of him, as he was as willing to join Sombra as he was to train Nagisa Shiota over the same hope that either would get rid of him.
At a very young age, presumably at birth, Shukaku possessed him, and used him as his vessel to escape the fate of most other Nine-Tailed Beasts. When this happened, he decided to run away from his tribe because of fear Shukaku would harm them if they were to find him. He found a shrine to make his home in the Surface World, and eventually became cold-hearted so no one would involve too deeply into the issue.
As he resided there, he found Nagisa wandering his forest, and at first he left him alone. When he found him again though, this time at his shrine, he immediately tried to kill Nagisa, and almost managed to do so until Nagisa's halo blade landed nearby, almost striking him. Gaara didn't stop him for staying through the night, but didn't let him go inside, despite it raining the following night. Next morning, he made it clear for Nagisa to leave, kicking him so he would wake up. However, he found Nagisa with Tirek, and after their talk, he made it clear that crying over it won't bring his friends back (believing they have all died). Nagisa asked him to train him, which at first Gaara coldly refused. He did agree though when he realized that Tirek's assault was going to affect everyone, including him.
He began training him over the following weeks, refusing to hold himself back just because Nagisa was his student. During this, he was asked about his demonic origin, as Nagisa could still sense a demon was nearby him, to which he simply replied that Tirek must still be watching them. When Tirek did show, he arrived and told him to get out, saying he could have Nagisa if he wanted him. However, Gaara was then attacked and pinned him down, and Tirek telling Nagisa he had to kill him to continue on. He was later saved, but questioned why Nagisa didn't kill him. He then asked Nagisa for the whole story in regards to the demons he had to deal with.
After the encounter, he allowed Nagisa to stay at the shrine until he was done with his training. During the practice, he and Nagisa overheard Yugure Kagayaki, Aladdin, and Gluttony nearby. Under Nagisa's insistence, he went in with him and drove Gluttony off. He was going to attack the other two, but held himself back and went back to the shrine. When he found Aladdin and Twilight at his shrine the following morning, he forcefully removed them from the area. When they came back, he told them that he did see Nagisa, but drove him off. He threatened them both that he would kill them if they stayed, but was stopped when Nagisa arrived.
After they stayed for the night, he went back with them and joined Team Harmony himself, but he made it clear he only went with them to get even with Tirek and nothing more than that. When he did arrive though, Gaara had to stay aside as Nagisa wanted to fight him one-on-one, him making sure no one else intervened. He was a bit disappointed it wasn't him who finished him off however.
After returning to his shrine, he was then confronted by Sombra, who he first tried to kill as a threat. Sombra offered him a chance of revenge on Nagisa, but Gaara only agreed for the promise of getting Shukaku out of his body. He went with Sombra over to Amestris in order to recruit Pride, Gaara defeating both his mother and Yoroi Kagayaki first.
He was taken to Tartarus sometime afterwards, though as time went by, he began to grow impatient with Sombra. He confronted him and demanded he fill his promise and remove Shukaku from him. This though didn't work.
Fed up, Gaara left Sombra, and returned to the Surface World in belief that it was taking too long. While back home, he found Aladdin, Grell Sutcliff, Heine Wittgenstein, Hoopa and Morgiana by his shrine again. Morgiana tried to get him to come back to the tribe, to which Gaara refused, and forced them all to leave him. He remained at his shrine until he was confronted by Heine, who he tried to kill, but was quickly defeated. He finally admitted to what he'd done, but in his weaker state, Shukaku took complete control over him. When Aladdin and Grell tried to get him out, he insisted that they leave before they get killed, just to hear that they weren't going to give up on him. Touched by both Heine's lesson and Aladdin's words, he finally took full control of Shukaku, and forced him into the sea, killing him.
Gaara fell unconscious afterwards, and upon waking up, decided to stay a member of Team Harmony, as they helped him remove the demon from him even after how he behaved to them all.
On his return to japan, Gaara took it on himself, and he actually returned to the Underworld in his own mission to get some information to aid the group in their final assault. He brought them to Celestia personally just as Team Harmony brought Lugia back.
In the final assault, he was in the assassination squad.