She is currently the captain of Fireside Girls Troop 46231.
Its members are Gretchen, Milly, Ginger Hirano, Katie, Holly, and Adyson, with which she is considered not only a leader, but a great friend.
Her friendship with Phineas has somewhere along the way deepened into love and he is now her crush.
It is unknown when Isabella joined the Lil' Sparks and later the Fireside Girls, where she worked her way up to become a troop leader.
Isabella is a very slender child with long, black hair with thin blue hi-lights and a rounded bottom, fair skin, and oval dark blue eyes and a flat top.
As a baby, she wore a pink t-shirt and had her hair tied up in a straight bun ("Agent Doof").
She is level-headed and responsible in the face of emergencies and dislikes being told what to do by anyone, with the exception of Phineas.
However, she does get jealous easily, and will get frustrated with someone that Phineas is giving a lot of attention to other than Isabella or when Phineas expressed seeing Meap as cute but not her ("That Sinking Feeling", "The Chronicles of Meap").
She comes over every day just to see him, as seen in the song "What'cha Doin? Isabella is sympathetic and would aid people who are in need of it.These instances include her immediately jumping on Baljeet's wager of squid versus Buford ("Voyage to the Bottom of Buford").