Phinella Flynn

Phinni is the gender-swapped version of Phineas.

The creation
When Ferb was insane, he zapped Phineas in his fortress. Isabella saw the lights and went to the forest. She later saw a girl in the forest, who is actually Phineas. He realized he was a girl and told Isabella immediately.

Name
It took time to think of Phineas' female name. Here's the names that Isabella suggested:

Phoebe

Phinabella

Phaira

Phi-Phi

Patricia (thought by Phineas himself)

Phinella (a.k.a. Phinni)

Phinni as an alternate person
Phineas and Isabella actually swapped bodies. Later, after Isabella (Phineas) released the trapped souls, she swapped back their bodies. Isabella resurrected with Phinni along.

Trivia

 * Phinni never appeared in anymore appearances.
 * The story she appears in was written in the mall.
 * She got an art concept but it was never shown in the public.
 * She is based on Marie Flynn, one of the most recognizable Phineas and Ferb OC.