For starters, I'm a huge Phineas and Ferb fan with a fading cartoon character crush on Isabella. There is no need for a background, nor is there one, so I shall promptly start on the object of focus - Isabella.
You stayed true to the animation style pretty much perfectly, to be honest. Her face and all her visible extremities are pretty perfect. Her smile is about as cute and adorably heartwarming as her character, as her pose itself is as well. Heck, you even included those faint white-ish lines in her hair. You also did incredibly well at accounting for any and all wrinkles, creases, and folds in her dress and princess hat sash. The addition of those few magic sparkles atop her hat and at the hems of her dress she's holding just complete the fantasy princess look quite nicely. Great job! Keep it up! God bless, peace be with you, and Godspeed in all your endeavors!