(!) Contains headcanon.

Spoilers!!

Kyoko Sakura (佐倉 杏子 Sakura Kyōko) is a veteran magical girl who appears to Sayaka and Madoka in episode 5, but not before making a brief appearance at the end of episode 4. She is introduced as an abrasive, fiery, and self-centered girl who fights only for herself, but later adopts a more sympathetic perspective.

In the spin-off manga Puella Magi Madoka Magica: The Different Story, Kyoko serves as the main protagonist alongside Mami Tomoe.

More informations about the character:

Kyoko_Sakura