Found in a cave below a hill in the Road of Sacrifices. From the Road of Sacrifices bonfire, follow the path toward a stone bridge and from there head to the left and down a narrow path that leads into the cave.
Isabella fights very similar to Maneater Mildred from Dark Souls and will often use a slam attack that leaves her exposed. Her swings are slow but do high damage and will break the player's guard swiftly if blocked, so it is advisable to evade them unless using a strong shield.
She is capable of buffing up her weapon for increased damage and to heal herself with each successful hit; however, this leaves her completely exposed for a couple of seconds and it's possible to stop her with repeated attacks.
Although risky, a quick way to defeat Isabella is to knock her off the path where she resides. She will occasionally fall off by herself if the player fights her close to the edge.