After you complete couple of routes, whenever you try to leave MC's room at any day, an event will start where player receives a package with a strange book. Then you can simply follow the walkthrough in the "tasks" menu.
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So I've done most of it and have even gone to reddit for the answer but I've ran into a wall. I've talked to Mothra and have also talked to to Reina and the librarian but I can't get the 3rd person. It says that it's Andre but every time I talk to her I only get the option to do the library and bathroom scenes and the people on reddit say she's not the one to talk to.