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Show other girls affection while Sami is present. When you are in a position where you can talk to other girls while she is around pick options that give Sami a ★. Filled in stars for Sami mean jealousy.

To add on to this, you can look at the conversation trees in the menu off to the side - if it's an option you've picked before, it'll show you symbols highlighting the changes in statistics.  Explore all options, and use that conversation tree tracker like a checklist to see what all the game has to offer!