Her wish was to heal Kamijou Kyousuke so he could play violin again, even though he was unaware of her feelings for him. When a demon horde was about to interfere with a recital of his, she defeated the horde, but suffered magical exhaustion to such a degree Sayaka soon succumbed to, killing her.
In her dying moments she won the love of Kyoukou, though.
Since her historical significance is not widely known, nor expanded upon by the surviving members of the Mitakihara Four, she is largely known for the impact her death had on Homura, since at this point the latter's memories of the previous loops were restored.
With the exception of a single picture in Kyouko's possession no image of her remains. The memory and impact of her remains, though, especially with and by Kyoukou, for example in naming places and buildings after her like the Miki Sayaka High School (attended by Okamura Yuu).
 Costume and Abilities
Sayaka's costume was a blue and white combination with a short, asymmetric skirt and a white, knee-length cape.
Somewhat related to her wish she had extraordinary regenerative abilities surpassing those of regular magical girls, allowing her to fight with more disregard to her personal safety, akin to a berserker state.