Upgrade her shield by purchasing better swords. Each upgraded shield will block more damage before breaking. The final upgrade, the Emerald Shield, will block all damage from the first hit. Shields reactivate after 10 seconds (in-game description says 30, but it is incorrect).

Whenever Isabel holds her sword, she will also hold out a shield on her left arm. The shield will block a certain amount of damage upon the first instance of damage taken. The better tier the sword is, the more damage the shield negates. The shield regenerates after 10 seconds (marked by the timer on top of the player screen).