Moe Chronicle

Each monster girl has an intimacy gauge, the set of 8 hearts indicating how much they like Io. Intimacy gauges can be filled by using the girls in combat, giving them presents, upgrading their rooms, performing Bumping Scratch, and using the "Divination" option at the inn for a specific girl and then taking her to the dungeon and floor described for a special event. Upgrading their rooms first will make the other actions increase the gauge faster. Filling the intimacy gauge unlocks an additional costume (1 heart), triggers event scenes (4 or 6 hearts) and earns you rare items (8 hearts). This trophy will unlock when you completely fill all 50 monster girls' intimacy gauges. This will require some grinding and will probably be one of the last trophies you unlock.

CO Mel on the GameFAQs forum has created a Word document listing the best presents for each girl, and aspreadsheet showing the appropriate Bumping Scratch spots for each girl (2nd tab).

If you have all of the girls' hearts filled but the trophy still does not unlock, make sure you have seen everyone's Divination scene in the dungeons, as they appear to be required to obtain the final event at the inn, even if the intimacy gauge is full.