As Gov. Eric Greitens of Missouri faces a criminal trial next week for a felony charge of invasion of privacy, he has sought to prevent the woman at the center of the case from testifying in open court.

A judge in St. Louis denied the request on Monday, saying that he would not bar her testimony — an account that could be crucial to prosecutors’ contention that Mr. Greitens took an explicit photograph of the woman without her consent.

Mr. Greitens’s legal and political fate could hinge on the account from the woman, who had sexual encounters with Mr. Greitens in 2015, before he was governor. So far, she has remained anonymous, though she has revealed details of her relationship with Mr. Greitens under oath to legislators in Missouri.

Here is what we know about the woman, who is identified in court filings only by her initials, K.S.

How did K.S. and Mr. Greitens meet?

K.S. is a resident of the St. Louis area who works as a hairdresser. A mother of two, she was separated from her husband when she had her first sexual encounter with the governor. She is now divorced.