Steve did not go with Locke, he stayed with the others as seen in Season 4 final. It is most likely therefore that she is NOT Blonde Casualty or the Red-Haired Woman.
Darlton said that Steve went with Locke. I can't remember the exact day, but it was in a podcast shortly following the Season 4 premiere. Why did we see him on the beach in the season 4 finale then? My guess is that he either left Locke's camp before the mercenaries got there or he avoided the mercenary attack and made his way back to the beach.
Apparently, according to the entry on Steve Jenkins, he is one of the people that was on the zodiac with Daniel when the Island moved. Since Sawyer insinuated Tracy was sleeping with Steve, and there was only one woman on the zodiac with Daniel when the Island moved, I suggest that this woman is Tracy.