Sayaka Miki

"Atashitte honto baka (Japanese- I'm such an idiot)"

- Sayaka Miki reflects on her life shortly before she dies

Sayaka Miki was a normal teenager in the Japanese city of Mitakigahara until she and she followed her friend Madoka Kaname into an abandoned mall, led by a mysterious voice. The two of them find Akemi Homura chasing after a mysterious white creature similar in appearance to a cross between a cat and a fox. Sayaka drives off Homura with a fire extinguisher but both of them inadvertantly enter a mysterious, psychedelic-looking "pocket dimension" and attacked by monsters that have been described by fans as "demonic Pringles".

Sayaka and Madoka are rescued by girl capable of materializing precussion rifles out of thin air. The girl introduces herself as Mami Tomoe, a "magical girl" with the duty to fight monsters called "witches", and the mysterious creature as "Kyubey. The "pocket dimension" they entered was the "barrier" of a "familiar", the offspring of a witch.

Kyubey asks them to make a contract with him and become magical girls. Mami however suggests that they first accompany her on a few of her "witch hunts" to see what the job is like. However, on their second "witch hunt",  Mami is killed when the "witch" literally bites her head off, causing Madoka to fall into despair. 

Later, Madoka discovers her friend Hitomi in an unnatural state, discovering a curse mark known as a "witch's kiss" on her neck. Madoka follows Hitomi and dozens of other people into a warehouse, and discovers they intend to commit mass suicide by gassing themselves with chlorine gas made by combining bleach and ammonia.

Madoka grabs the bucket of bleach and tosses it out the window of the warehouse. This angers the people influenced by the witch's kiss, and is force to flee into a side room, where she is transported into a "witch's barrier".

Things look hopeless for Madoka until Sayaka rescues her, revealing she is has in fact made a contract with Kyubey and become a magical girl. Sayaka manages to kill the witch, but refuse to tell Madoka what she wished for, simply speaking of her desire to be a hero, declaring that she will protect the city.

It turns out Sayaka wished for her crush, a boy in her school named Kyousuke Kamijou to be healed. Kamijou was a skilled violinist until his hand was wounded in an accident, and was told he could never play again. Sayaka spent much time by his side in the hospital and saw how depressed he was about the loss of his greatest talent.

Sayaka soon after meets Kyouko Sakura, a dark, cynical magical girl who becomes frustrated by the Sayaka's ideals of justice and heroism and calls her a fool for using her wish to help another. Kyouko and Sayaka get into a fight, and Kyouko injures Sayaka, causing her to discover her regenerative powers, which she by wishing for someone to be healed. Nonetheless, Kyouko outfights and almost kills Sayaka, were it not for Akemi Homura breaking up the fight.

Shortly afterwards, the first hint that Kyubey may have malicious intentions (for full details see Akemi Homura's article) occurs Kyubey reveals to Sayaka that the gem he gave when she became a magical girl, her "soul gem" is  just that, literally, Kyubey placed her soul, as well as her life force into a gem. Sayaka is infuriated by this, but Kyubey simply states that this makes her stronger, as she feels less pain and does not sustain bodily injury.

Sayaka keeps on fighting witches, but as she does so, things go poorly in her relationship with Kyousuke, who falls for Sayaka and Madoka's friend Hitomi instead. At around this time, Sayaka discovers she can reduce the pain she feels in battle by placing her "soul gem" somewhere further away form her. During this time, Sayaka also makes up with Kyouko, who reveals that she actually has similar tragic past. However, at the same time, Sayaka neglects to cleanse her "soul gem" of the darkness that builds up inside it as she uses her magic, and eventually  descends into despair, transforming into a witch herself.

Abilities
Sayaka's primary magical weapon is a sabre or cutlass. Sayaka has a seemingly endless supply of these swords and uses them as both slashing and throwing weapons. The series production notes reveal that, though she is never seen using this ability, Sayaka can also turn her sword into a chain whip in a manner similar to Kyouko Sakura's spear. The series guidebooks says that she has another unseen ability- the ability to launch the blade of her swords like a ballistic knife. Once embedded in the enemy, the blade will explode like violent. Sayaka can then replace the blade by inserting a magazine into the grip of her sword, creating a new blade from liquid metal.

Sayaka also possesses superhuman speed, sometimes appearing as little more than a blue blur. In addition, she has regenerative abilities that allow her to survive a blow that according to Kyouko Sakura, should have "put her out of action for months"