This is Louise Annabelle Crossby from the alternate dimension.
Unlike Louise from the first dimension, Louise is much more sensible. She will do anything to help others, so they don't have to go through what she went through.
At a young age, Louise's parents were taken away by Doofenshmirtz. At the age of 8, she joined the resistance and was immediately put to work by Candace. Louise currently is the general of a member of the army. In PF2D, she secretely helped Phineas and Perry by rolling the baseball bat to Perry. She appears in Phineas and Ferb: Back to the 2nd Dimension where she, along with Marly Spears appear as major characters.
Louise has a crush on Phineas from the second dimension. In a "deleted scene" from PF2D shown in PFB2D, Phineas and Louise are fighting side-by-side and he tells her to go stop Doofenshmirtz. She doesn't want to leave him "to be killed" but Phineas orders her to leave. Phineas and Louise hug and Louise runs off to help Phineas (1st dimension) and Perry (1st dimension). Later, in PFB2D, Phineas and Ferb go to save first dimension Phineas and Ferb and Louise finds them. She tells him it's not safe and not to go but Phineas insists. They hug and Phineas leaves.
Louise and Ferb have a good relationship, not as good as Louise and Phineas, but better then Louise and Doofenshmirtz. Louise worries about Ferb's safety when he and Phineas go to save the first dimension Phineas and Ferb. She holds his hand for a short moment in PFB2D, then runs off.
Doofenshmirtz is Louise's lifelong enemy. She hates him and, according to her, "wishes he was dead".
Louise likes Phineas from the first dimension, but mostly just enjoys his company. She assists him in the ending of PF2D, as said by her in PFB2D and hugs him at the end of PFB2D.
Louise enjoys playing with Perry and helps him defeat Doofenshmirtz in PF2D.
Louise-2 and Louise-1 don't get along, but Louise-2 tries to talk some sense into her counterpart. They team up to save Phineas and Ferb in PFB2D.