I mean, Monika herself shows signs of depression. In one of the text files that spawn during the second act she seriously ponders killing herself (and no one pushed her to think like this unlike Sayori and Yuri who was influenced by Monika). She even says you "saved her life" because she would have ended it if the player hadn't come. Plus, despite all the power Monika has she is actually incredibly meek and bows down to both Natsuki and Yuri whenever they stand up to her or get into fights and is never shown being assertive onscreen (she reserves all the messing with the game to offscreen). ---