After completing both Siri and Eleni's good routes (and replaying Elani's to get the secret ending) I am unable to get to Siri's secret ending (and his 4th photo). Is there some additional thing I need to do?
I got the 4th image from the time I first encountered him. Make sure not to choose him for anything until you have atleast 10 strength. I did this by trying my luck eating the random mushrooms for strength boosts. Then meet him and go through the dialogue in that unknown part of the school you somehow wander into. Pick the top option that sounds the most confident and you'll get the image.