Strawberry Vinegar (EP)

You'll get this trophy the first time you purchase Onigiri for Licia, which happens to only occur during one Ending; during the latter half of the Lotus Blossoms Ending.

See the Platinum Walkthrough [LINK] or the Lotus Blossoms trophy for more details and exact steps.

After you choose not to go to the Festival and do homework instead, choose to ask Dad to help Licia, and then soon after, choose to console her. The next day at breakfast, Rei wants to head to class and not be late, so choose to buy lunch for her instead of making it. That will then lead to having Onigiri for lunch and the trophy will pop then.