Aerith is a character who lives in Traverse Town in Kingdom Hearts and Kingdom Hearts: Chain of Memories, and in Hollow Bastion/Radiant Garden in Kingdom Hearts II. Like many of Sora's allies, she originates from the Final Fantasy series.
Aerith, as an ally of Leon and Yuffie, is one of the three core Final Fantasy characters of the series, and takes an active support role with them in the defense of Hollow Bastion in Kingdom Hearts II. She also plays a part in Cloud's story, though does not directly assist him as she does her other allies.
Aerith's name was never spoken in-game even though her name is written in the script. This is due to uncertainty during the game's development whether her name would be translated as "Aerith", or as the original translation "Aeris" that appeared in Final Fantasy VII. The name "Aerith" would later be used in related releases of Final Fantasy VII, making the original "Aeris" considered to be a mistranslated name.
Aerith, along with Cloud, Tifa, Yuffie, Cid, and Leon were originally set to appear in Kingdom Hearts Birth by Sleep. But in the final release they were removed from the game because, according to Nomura, there were too many plot lines already, and it would have gotten confusing.
 Kingdom Hearts
Aerith is encountered by Donald and Goofy on the first trip to Traverse Town, where she explains the nature of Ansem Reports, as well as the Keyblade to them. She had a few more appearances along with Leon and Yuffie. Later, she showed up in Hollow Bastion after Ansem, Seeker of Darkness disappeared, and spends her time studying the Heartless's research in Hollow Bastion's library. If you talk to her a multiple number of times she will give you the missing Ansem Reports that Maleficent had and Curaga, the strongest Cure spell in the game.
During the end credits she was seen with Leon and Yuffie inside the Hollow Bastion library, and was the first to greet Cloud when Cid returns him to their world.
 Kingdom Hearts: Chain of Memories
A memory-based version of Aerith appears in the Traverse Town floor in Castle Oblivion, along with Leon, Cid, and Yuffie. At first, she, like the other characters, expresses disbelief that she seems to know Sora despite not remembering him. After speaking with Sora one-to-one, however, she comes to realize that she is just a projection based off his memory of her. She along with Maleficent in Riku's trek through the castle, are the only ones to realize this. She tells Sora to be on his guard. However, it seems that Sora is the only one who can see her in the first place, as Donald and Goofy assume that she had gone back with Leon, Yuffie, and Cid already.
 Kingdom Hearts II
Aerith is a member of the Hollow Bastion Restoration Committee. In Kingdom Hearts II, she wears a modified version of her Before Crisis outfit, though wearing a pink skirt instead of blue. She helps during the invasion of Hollow Bastion by healing Yuffie and presumably continues to aid other characters or fight against the Heartless. Aerith assisted Sora later in the game during his movement against the Master Control Program by downloading the deletion program while guarding the computer room.
In her last appearance of the game, during the ending credits, she is seen sitting on a ledge and looking up towards the Radiant Garden's sky, possibly wondering what happened to Cloud and Sephiroth after their fight at the Dark Depths (assuming the player has completed the sidequest involving them). Throughout the story, she encourages Cloud to do what he must to fulfill his goals. Aerith is sure that Cloud will find his light, which will lead the way back to them.
In Kingdom Hearts, Aerith bears a strong resemblance to her original appearance in Final Fantasy VII, though she lacks her short, maroon overcoat and her thick, metal wristbands are replaced with what looks like coiled wire. She also has a loose, lilac belt around her waist with black string tied into a bow on it that was not present in her original design. Aerith wears a pink dress with magenta lining and buttons going down the front, all of which are buttoned except for the last two. She has a black string tied in the same fashion as the one on her belt around her neck like a necklace.
Her eyes are green and her hair is brown. Aerith's hair is long and styled into a braid down her back, with curls on either side of her face. Her bangs frame her face in a very similar manner to Sephiroth's own bangs (this is likely because at one point during the developing stages of Final Fantasy VII, Aerith and Sephiroth were planned to be siblings). She also wears brown boots.
In Kingdom Hearts II, Aerith's look changes, now somewhat resembling the way she looked in Crisis Core -Final Fantasy VII-, albeit with several differences. Her eyes and ribbon are much brighter, while her boots (not the sandals she wears in Crisis Core) are paler. Aerith now wears a carmine pink and white top held up by a string that wraps around her neck. Underneath this top, Aerith wears a pink dress with a horizontal white stripe near the bottom. She wears yet another dress underneath the first one, this dress being white and slightly longer than the first.
Aerith, unlike most other Final Fantasy characters, has not yet been either a combat ally or opponent for Sora, or seen to engage directly in battle against an opponent, and so her combat abilities, if any, are unknown. Aerith does seem able to cast magic, however, as she casts a Curaga spell on Yuffie during the invasion of Hollow Bastion, and also upgrades Sora's Cure spell. This seems roughly in-line with her Limit breaks, centered around healing, in Final Fantasy VII. Despite never physically being seen in battle, she is implied at one point to have held off waves of Heartless spawned by the MCP (coldly spurning Leon's concern over whether she would be able to handle it by herself), hinting that she does possess combat abilities.
She carries no visible weapons at any time, unlike most other Final Fantasy characters, but it would likely be a quarterstaff as per her original incarnation.
Aerith Gainsborough debuted in the original Final Fantasy VII as one of the lead female protagonists alongside fellow party member, Tifa Lockhart. Though seemingly fitting the role of a White Mage in this game, Aerith uses an actual melee weapon in the form of a staff rather than a scepter. Aerith is the last member of an ancient race known as the Cetra. She makes a peaceful living as a flower girl with some form of communication with the Planet and develops a short-lived relationship with Zack before meeting Cloud. Aerith's death at the hands of Sephiroth went down as one of the most famous events in RPG history, and it is this that makes Cloud more determined than ever to fight and defeat Sephiroth once and for all.