In an interview, the 26-year-old flight attendant known as Maria W told German newspaper Bild that when she heard about the crash she recalled Andreas Lubitz telling her last year: "One day I'm going to do something that will change the whole system, and everyone will know my name and remember."
The black box voice recorder indicates that Lubitz, 27, locked his captain out of the cockpit on Tuesday and deliberately flew Flight 4U 9525 into a mountainside, French officials say, in what appears to have been a case of suicide and mass killing.
German prosecutors revealed that searches of Lubitz's homes netted "medical documents that suggest an existing illness and appropriate medical treatment", including "torn-up and current sick leave notes, among them one covering the day of the crash".
They did not specify the illness.
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According to Bild, the young woman, who was "very shocked", flew with Lubitz on European flights for five months last year, during which time they are believed to have been romantically involved.
If Lubitz did deliberately crash the plane, "it is because he understood that because of his health problems, his big dream of a job at Lufthansa, as captain and as a long-haul pilot was practically impossible", she told Bild.
Bild earlier reported that Lubitz sought psychiatric help for "a bout of serious depression" in 2009 and was still getting assistance from doctors, quoting documents from Germany's air transport regulator.
Lufthansa CEO Carsten Spohr said that Lubitz had suspended his pilot training, which began in 2008, "for a certain period", before restarting and qualifying for the Airbus A320 in 2013.
Half of the 150 victims of Tuesday's disaster were German, with Spain accounting for at least 50 and the remainder composed of more than a dozen other nationalities.