To unlock this trophy you need to romance one of the female confidants. This can be done with Ann, Makoto, Futaba, Haru, Kasumi, Takemi, Ohya, Kawakami, Hifumi, and Chihaya.
When you reach Rank 9, you'll be able to choose whether or not you want to romance them. Joker will think "I should choose my words carefully..." and your following responses will dictate whether or not you start a relationship. The game is fairly obvious as to which is the platonic and which is the romantic response so there shouldn't be too much confusion.
It's worth noting that romancing one confidant doesn't prevent you from romancing any others; you can romance every confidant in one playthrough with no repercussions.