Unlike her 1st Dimension counterpart, Isabella is a tough, no-nonsense, and down-to-earth girl; she even says her catchphrase in a tough voice in the film. She also appears to have a sense in fashion, as stated by both Candaces. In the episode, she seemed to get more toward her 1st dimension counterpart, most so in the ceremony, when she wore a pink bow, Isabella-1's signature hair item.
It should be noted that considering how bad the situation in Danville and Tri-State Area during Doofenschmirtz's reign, combined with the fact where Pinky was missing, Isabella's tough and no-nonsense were likely result of constant battles against Doof's forces that hardened her qualities. Even though she returned to the original personality that made her closer to her 1st dimension counterpart, her hardened side still remained.
Isabella is physically identical to her 1st Dimension counterpart, although her eyes are slightly more oval-shaped due to her formerly serious personality. She has only been seen in a silver shirt with torn sleeves; brown pants with diamond-shaped silver patches on the portion that covers her kneecaps; black shoes; gray gloves; a dark-green beret with a drooping dark-green bow on top; a brown strap slung over her left shoulder, holding what appears to be rolled-up strips of paper; and a tan badge with a green circle at the center, with a red lightning-bolt insignia running across it (most likely the Firestorm Girls symbol). However, she is seen with a pink bow at the end of the episode, reflecting her happier life. She has black long hair.
Isabella meets Phineas-1 and Ferb-1, along with their 2nd Dimension counterparts, when they stumble upon the Resistance headquarters after falling down a chute (most likely Pinky the Chihuahua's former lair) along with Buford, resulting in their capture by the Firestorm Girls. Buford's presence gets her to release the four. She is present when Dr. Baljeet opens a portal to the 1st Dimension, which is later disrupted and closed by Candace-1. Isabella later accompanies Candace, Phineas-1, Ferb-1, Candace-1, and Buford to Doofenshmirtz Evil Incorporated aboard mine carts in their quest to save a captured Perry-1, to which the group finds out that Phineas and Ferb stowed away.
Accompanying Phineas-1, Ferb-1, Candace-1, and Buford to DEI, they are nearly captured by Doofenshmirtz and Doofenshmirtz-1, but Perry-1 manages to escape and the group reunites with Candace, Phineas, and Ferb. A resultant chase by the Norm bots forces Candace to separate the mine cart containing the 1st Dimension group from the others, to which Isabella looked genuinely concerned about.
She is later among the Resistance members that congratulate the 1st Dimension group for defeating Doofenshmirtz.
She returned in the episode, Tales From the Resistance: Back to the 2nd Dimension, where she is seen in a happier tone following Doofenshmirtz's arrest. However, Isabella and the kids will now have to deal with a gang of O.W.C.A. agents, who were captured and transformed into ruthless cyborgs by Doofenshmirtz's wife Charlene. Also in a more a darker tone, Isabella learns that her pet chihuahua Pinky (now known as Chihuahuaborg) is among the cyborgs, much to her horror. In the end, Isabella manages to free Pinky of his evil programming and reunites with him, and she and the other Firestorm girls later attended the reunion of all returned O.W.C.A agents following the Doofenshmirtz family's escape.