Mr. Bankman-Fried, 31, is accused of misusing billions of dollars taken from customer accounts and faces eight counts of fraud and election law violations. His spectacular downfall, which sent FTX and Alameda into bankruptcy, transformed Ms. Ellison from a powerful — yet relatively private — figure into a target of tabloid speculation. In December, she pleaded guilty to fraud charges and agreed to cooperate with the federal prosecutors investigating her former boyfriend.

His case is speeding toward a courtroom showdown in Manhattan. Two other top FTX executives, Nishad Singh and Gary Wang, have also pleaded guilty and agreed to cooperate. In June, after weeks of legal wrangling over the charges against Mr. Bankman-Fried, the judge in the case set a brisk schedule for the run-up to the trial, asking prosecutors to come up with a witness list and produce other final materials. Prosecutors are expected to begin preparing at least some witnesses in August, two people with knowledge of the matter said.

As Mr. Bankman-Fried’s sometime-girlfriend and one of his earliest hires, Ms. Ellison had unique insight into the FTX founder. She also recorded many of her thoughts in writing, making observations about her personal and professional life in a handwritten diary and on Google documents that have circulated among lawyers involved in the case, according to documents reviewed by The New York Times and four people familiar with the investigation.

The documents, which have not been previously reported, offer new insight into Ms. Ellison’s psychology during the final months of FTX. Ms. Ellison, now 28, was a prolific writer whose Tumblr posts about Harry Potter and Jane Austen have been widely dissected. But the Google documents are more personal and raw, with some directly addressed to Mr. Bankman-Fried, illustrating the complexity of their relationship and her ambivalence about Alameda.

In one Google document addressed to Mr. Bankman-Fried in April 2022, Ms. Ellison wrote that an earlier breakup with him had “significantly decreased my excitement about Alameda.” Life at the hedge fund, she added, “felt too associated with you in a way that was painful."