“You are who you are because you forgot who you were.”
The Wall and the Wing by Laura Ruby doesn’t feature a titular girl per se (although she is a “Wall” – you have to read the book to discover what that is), but the protagonist’s name is simply “Gurl” which is interesting in itself.
Gurl lives in an orphanage, abandoned as a baby with no name – so the unpleasant matron simply calls her what she is (no explanation of the spelling). She lives in a world much like our own, but where most people can fly to some degree or another, although Gurl cannot. One night she escapes to explore the city, and discovers two things – a cat (rare in this world) who follows her home, and the startling fact that she is capable of becoming invisible.
Gurl names the cat Noodle (strangely the second animal companion named Noodle I have encountered recently) and hides her away, but the matron captures her and uses her as leverage to make Gurl steal things for her. In the meantime, she begins making friends with Bug, a fellow orphan who discovers her secret. Together they rescue Noodle and escape, helped along the way by a crazy Professor and a mysteriously helpful stranger. They find out that Gurl is being sought by a gangster named Sweetcheeks (the real adversary, more frightening than the matron) who wants an invisible girl for his crime business. His minions are the rat-like man-creatures who live in the sewers (although they turn out to have an unexpected soft spot).
There appears to be a companion betrayal, but it is complicated and he remains on her side in the end. There is a theme of the orphans forgetting who they are. There is even a “return home” in the middle, although Gurl doesn’t realize it. She does defeat Sweetcheeks, but not really on her own in a final standoff. However, in doing so she discovers her true identity, and her place in the world.