A boss encountered during the main story. By playing through the main story, this cannot be missed.
The Rogue is a giant alligator that is fought shortly after recruiting Karine. Using Marine's "Red Orb" skill can help, as well as summoning Jeanne to help. Use Benoit's Iron Fist and Wind Fist abilities, and attack normally with Karine. For healing, you can get Jeanne to help or use items. Both Marine and Benoit also have a healing spell in their skill menu as well.
If you are a high enough level, the Rogue shouldn't be too much of a problem.