Hello Xth Squad,
Rare items in this game are those with yellow names, which some people might think of as "unique" items based on their experience with Demon Gaze. To unlock this trophy you'll need to collect 30 different rare items, duplicates of the same rare item will not count towards this trophy, and depending on your luck with drops you can easily obtain this during the first semester.
If there is anything I can do to make this solution better please don't hesitate to let me know.