There are two opportunities to get Siren in the game. The first comes very early when you're in Dollet. After fighting Biggs and Wedge, you'll then fight a boss named Elvoret. Draw the GF from this boss.
If you happened to miss it the first time around, you can draw Siren from a boss named Tri-Point in the final area of the game.
If you miss it a second time, you'll need to start a new game.