At her parents’ home in a working-class Parisian suburb, Ms. Madani was a self-effacing teenager suffering from depression who had become radicalized under the influence of an older friend. She never followed through on plans to go to Syria .

On social media and messaging platforms, though, Ms. Madani was Abou Junaid, or Abou Soulayman, using various Arabic pseudonyms to pose as a fearless Islamic State fighter who had returned from Syria to recruit young men for attacks in France.

In 2016, she turned to women who were sympathetic to ISIS, interacting with at least 12, according to court files. After she approached Ms. Gilligmann over religious quizzes on the live-streaming app Periscope, the two sent each other thousands of messages over three months, developing an intense virtual relationship. When they spoke by phone, Ms. Madani modified her voice. She sent pictures of her stepbrother to make her claims more credible; she also ordered underwear for Ms. Gilligmann, who sent back intimate pictures.

“I was blinded; I would only think about Abou, Abou, Abou,” Ms. Gilligmann told the court. She said she had wanted to divorce her husband and marry Ms. Madani’s fake persona.

Ms. Madani found the omnipotence exhilarating, said one psychiatrist who examined her. Another warned judges against the “simplistic view” that one woman had been the puppeteer of the other.

The two women met in person at a fast-food restaurant near Paris a couple of days before the attempted attack, Ms. Madani posing as Abou Soulayman’s sister. Their conversation that day was at the heart of the trial: Who suggested the plan for a bombing near Notre-Dame? Ms. Madani argued that they vaguely discussed an attack. She later said that she wanted to “die as a martyr” by staying in the car, but that Ms. Gilligmann had discouraged her from doing so.