Isabella Garcia-Shapiro

Isabella is a girl that lives across the street from Phineas and Ferb. She is hopelessly in love with Phineas, it's really quite sad actually.

Theories

 * Isabella might be an alien because she wears a lot of pink and have an excess love for Phineas.
 * Isabella might be Major Monogram's daughter or stepdaughter. Monogram's wife is all about Mexico and stuff. In "Doof Dynasty", Major Monogram is also depicted as Isabella's father. However, Isabella met Major Monogram many times without mentioning that he's her father, such as "Undercover Carl".
 * her bio father is not mentioned but her mother has been seen with a man (tall black hair) in a the background of some episodes he might not be her father but comes close.