You must interact with and listen to Maechen elaborately explain several background elements. When interacting with him, it's better to just not touch the controller and let him divulge everything he knows, for fear of nulling the trophy.
Ch.1: You can find Maechen on Mushroom Rock Road after being introduced to the Youth League.
Ch.3: After interacting with Logos and receiving the Gaol Sphere, Maechen will appear in the room.
Ch.4: You can listen to Maechen through the Comm Network. After the concert, you will be able to find him onboard the Celsius's Bridge.
Ch.5: Maechen will be ready for his final tale in Zanarkand. This will be the last story, and you'll get the trophy after hearing it all.