Isabelle is a powerful necromancer's pet, belonging to Xanlosch. They are the target of the quest Risk Life for Limb, received from Jonelath if he was chosen for your Battle Tower. Isabelle can be found at the center of Stone's Flying Fortress in the Orobas Fjords, near the anti-dragon field generator. You'll need to use the teleporter in the west-central part of the fortress and pass through the "inner passage" area to get there, since you cannot fly. A book dictated by Xanlosch while feeding Isabelle, Creature Training Notes, can be found on the ground near a chest in the fortress's inner passage.