Sasa Yuuki is a magical girl from Kazamino City seeking to claim new territory in Mitakihara City. Her wish was for people who are better than she is to follow her every command; in result, her magic is brainwashing, which includes the ability to control witches.
Sasa is a girl who at first appears cheerful and dependable, but this side masks an incredibly vicious, haughty, and possibly even vengeful side of her. She has a tendency to laugh madly at times, and generally does not act very serious. Sasa is also extremely manipulative, and is not above resorting to tricks such as begging or feigning surrender/cowardice in order to get herself out of difficult situations. She is someone who has killed many, as she seeks out victims to feed the witches under her control.
After Oriko reveals to Sasa that magical girls become witches, she is horrified, crushing her soul gem while screaming how she doesn't want to become a witch, immediately killing herself.
(Source: Puella Magi Wikia, edited)