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Finish both Eleni and Siri's route with a success ending and start Eleni's route again. At the end when it asks you if you've seen Siri before, select yes.

Go back to Eleni's route and select yes if the game asks you if you know Siri

I'm pretty sure Siri's good ending is Eleni's secret ending because it was listed as Eleni and Siri's good ending. I might be wrong though

All you have to do is get her success ending along with Siri's success end and then select the option yes at the end of Eleni's route when the game asks you if you know Siri or not.