Polly Stone's powers emerged when a devastating earthquake shook Culverton. All around her, in every direction, there were people screaming for help. She whirled this way and that, not knowing where to start, how she could possibly pick one sufferer from the wailing mass of terrified humanity. The trauma threw her out of synch with the timeline, and caused her to split into multiple copies of herself.
Shimmerstorm, as she became known, can create solid doppelgangers capable of performing simple tasks. They were able to dig people out of the rubble in different parts of the city, for example. She also spawned phantom versions, which had no physical form but could locate survivors and where possible guide them to safety.
It was during her rescue work in the earthquake's aftermath that she met each of the Emergents who went on to form The Upstarts with her -- a name the media invented, since the team had helped deal with that local crisis while more established heroes were elsewhere grappling with global matters.
Due to her strange relationship with the timeline, she receives glimpses of the future. But this temporal perception also makes her feel isolated from the world around her. She's completely relinquished her identity as Polly Stone, and is now only Shimmerstorm.