Aerith

Aerith is a character who lives in Traverse Town in Kingdom Hearts and Kingdom Hearts: Chain of Memories and Hollow Bastion/Radiant Garden in Kingdom Hearts II. Like most of Sora's allies, she originates from the Final Fantasy universe. Aerith is voiced by Mandy Moore in Kingdom Hearts, Mena Suvari in Kingdom Hearts II, and Maaya Sakamoto in the Japanese versions of both games.

An ally of Leon and Yuffie, Aerith is one of the three core Final Fantasy characters of the series, and takes an active role with Leon in the defense of Hollow Bastion in Kingdom Hearts II. She also plays a large role, maybe as/with Tifa, in the backstory of Cloud.

Kingdom Hearts
''She lost her home to the Heartless at a very young age. Beneath her gentle disposition lies a strong will and a firm sense of duty. Many are naturally drawn to her. Aerith works with Leon and Yuffie to learn more about the Heartless and the "key". "Final Fantasy VII"''

Kingdom Hearts: Chain of Memories
''A young woman who fought the Heartless. Her calm demeanor hides an unbreakable will.''

Aerith also has a mysterious sense of truth, and is aware that Traverse Town and it's inhabitants are a figment of Sora's memory.

Kingdom Hearts II
''A young woman whose quiet eyes belie an unbreakable will. Like Leon, she met Sora while living away from her hometown, and helped him now and then on his journey.''

Now Aerith works with Leon and Yuffie to restore the town to its past splendor.

Kingdom Hearts
Aerith is encountered by Donald and Goofy on the first trip to Traverse Town, where she explains the nature of Ansem's Report, as well as the Keyblade to them. She had a few more appearences along with Leon and Yuffie. Later, she showed up in Hollow Bastion after Ansem dissapeared, and spends her time studying the Heartless's research in Hollow Bastion's library. Talking to her will allow Sora to learn "Curaga", the strongest cure spell in the game. During the end credits you can see her inside the library, being reunited with Cloud.

In a scene exclusive to Kingdom Hearts: Final Mix, Aerith senses Cloud was at Olympus Colosseum, and alludes to Cloud seeking Sephiroth when she discusses the matter with Sora.

Kingdom Hearts: Chain of Memories
An image of Aerith appears, along with Leon, Cid, and Yuffie, in the memory-based Traverse Town in Castle Oblivion. At first, she, like the other characters, expresses disbelief that she seems to know Sora despite not remembering him. After speaking with Sora, however, she comes to realize that she is just a projection based off his memory of her. She and Maleficent, in Riku's Reverse Rebirth, are the only ones to realize this.

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, only with a red skirt instead of a green one. 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 MCP by downloading the deletion program.

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 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, and she is sure that he will find his light and find his way back to her.

Abilities
Aerith's abilities in combat do not seem particularly proficient, as she is to date, the only Final Fantasy character that is not seen in battle. Furthermore, she possesses no obvious weapons. Aerith does seem able to cast magic, however. She casts some sort of Cure spell on Yuffie during the invasion of Hollow Bastion, and is also able to upgrade Sora's Cure spell. This seems roughly in-line with her role as a healer and White Mage in Final Fantasy VII.

Trivia

 * Aerith's clothing in Kingdom Hearts and Chain of Memories is very similar to the one she wears in Final Fantasy VII, minus the jacket. A belt is also added but other then that, no dramatic changes have occured. In Kingdom Hearts II however, her outfit undergoes a major change and resembles the one she wears in Before Crisis, only to be red instead of green.
 * She is one of few characters whose name is never spoken during cutscenes, only written in dialog boxes or menus.