Frisky Business (EP)

Serena is Balbina's sister and you will come across her in Chapter 9 but you need to date Lucy in order to get the "cheater cheater" reaction. This trophy is part of the Run #1 Walkthrough found here.

Note that this is one of the trophies that is a little glitchy and may not unlock right away. If you have finished all three runs of the game required and it has still not unlocked, replay Run #1 up to (and through) Chapter 9 and the trophy should unlock just fine.