Isabella is a CO of the 12th Battalion and one of the main protagonists of Advance Wars: Days of Ruin. She is a young girl who has no memory of who she is, but does have detailed knowledge about military information.
Isabella was discovered by Will barely alive and in some ruins. After being rescued and brought back to the 12th Battalion's camp, Will, Lin and Brenner learn that she is an amnesiac with no memory of even her name, but that she does have detailed knowledge about the Rubinelle Military. Because she has no memory of her name, Will opts to name her Isabella after a plastic Cattleya Isabella flower the two discovered in some ruins.
After Caulder preforms his "Fear Experiment", Brenner's Wolves learn about the virus the Creeping Derengea, or "The Creeper" and Isabella becomes infected with the virus. Brenner's Wolves later manage to locate a nuclear shelter constructed for Rubinelle's political elite in case of and all-out nuclear war with Lazuria after Isabella informed them of its existence. Discovering that they need a ten-digit passcode to get inside the shelter, the 12th Battalion loses hope until Will inputs a ten-digit number he heard Isabella randomly muttering; allowing them to get into the shelter.
Eventually, Brenner's Wolves learn who she really is: Lutaria, one of the cloned children of Dr. Caulder, an infamous scientist and the head of private military contractor IDS. As Caulder threatens Brenner's Wolves unless they turn her over, civilian opinion turns against her and Isabella, thinking that no one wants her, decides to leave camp and surrender herself to Caulder. Caulder places her in the laboratory on board the Great Owl and informs her that he never really wanted her back and only made the demand to see what the 12th Battalion would do in response.
Isabella then tries to connect with her "sister" Tabitha, only for her clone sibling to laugh at her and demean her, revealing to her that she is one of many expendable clone Lutarias. Tabitha then leaves the Great Owl and Isabella is left on board with her other "sister" Penny. As the war plane takes off, Will and the 12th Battalion show up to rescue Isabella. Convinced by Will that she is human and deserves to live, Isabella decides to rejoin the Brenner's Wolves. However, after the 12th Battalion forces defeat the IDS forces on board the plane, Caulder orders Penny to crash the Great Owl to kill the 12th Battalion troops, but Isabella convinces Penny not to listen to Caulder and to instead come with the 12th Battalion.
After the Great Owl lands, Isabella goes with Brenner's Wolves to pursue Caulder to the Nest to obtain the cure for the Creeping Derengea. During the final battle between the 12th Battalion and Caulder's IDS forces, Isabella gives the 12th the information needed to destroy the Nest: destroy the base's five laser cannons to cause its reactor to melt down and explode. After the Nest is destroyed, Isabella and the 12th Battalion obtain the cure and return to camp.
One year later, Isabella has become a part of the society set up by Brenner's Wolves. She told Penny to go look up at the sky, but Penny didn't like it very much due to it being cloudy. She later looks up at the sky with Will, and the two notice sunlight shining through the clouds.