The characters in John Carreyrou's Bad Blood are contemporary figures (mostly businesspeople, scientists, and investors). The author, a reporter for The Wall Street Journal, claimed to have interviewed over 150 people in writing his account. In his book, he seeks not only to deliver the facts, but to investigate how Theranos deceived so many for so long.

The characters can be broadly divided into three categories: Theranos employees, investors, and whistleblowers.

Elizabeth Holmes is the nineteen-year-old Stanford dropout who founded Theranos (a hybrid of "therapy" and "diagnosis"). She sought to build a reputation to rival that of Steve Jobs, founder of Apple and Holmes's personal idol. The inspiration for her product stemmed from her childhood fear of needles and was fueled by her lust for power and money as well as her education and personal reputation.

Holmes became romantically involved with one Ramesh "Sunny" Balwani, then COO of Theranos. He filled many positions within the company with Indians who were happy to comply with the policy of silence insofar as they were dependent on their jobs at Theranos for their visas. Balwani was charged alongside Holmes for fraud by the SEC.

Tim Kemp was in charge of bioinformatics at Theranos (and formerly at IBM) he was chiefly responsible (at Holmes's behest) for faking results in order to garner financial support for Theranos.

The investors included Channing Robertson, a professor of engineering at Stanford and mentor to Holmes. Others included retired Marine Corps General Jim Matthis, who was attracted to Holmes' innovative idea and served on the board of Theranos. Also among the distinguished board was Henry Kissinger, former US Secretary of State.

The writer himself was prominent among the whistleblowers. He was born in New York and raised in Paris and joined the Wall Street Journal's investigative reporting team in 1999. When assigned to the Theranos case, he got his lead from the personal blog of one Adam Clapper, a practicing pathologist who questioned the scientific merit of Theranos's discoveries when the company was featured in a Fortune magazine in 2014.