From what I've seen. After the last fight with Sexton you need to go to te port city and you'll get an event. Still, I'm not 100%. Only a few people on the board have gotten her and there's been a lot of confusion about just how to get her.
When I grow up I want to be Dracula or Snoopy or maybe even a crayon.
If you're following the guide, it's right after you defeat all of the human genomes. It's not always right after Sexton, depends on what order you kill the genomes (total of 6 battles). You need Issac, but not sure if there's any other requirements. After you've done that an event appears at Gulvia where you get her and her scythe.
Should happen right before the battle:
Trace Route Event Battle 1 (Lv. 65) Event Battle 2 (Lv. 70) Fight Edovardo, Hermes, Helmuuto Talking Event at Noashulan Enter Noashulan Again for another event -For this event, you pick who Zelos talks to.