SOLVED. Book about a girl who disguises herself to secretly help others. [s]

[Serena]

I think it was set in an inn. The heroine of the book finds a costume hidden away that she uses to disguise herself. She sneaks out at night and performs deeds like Robin Hood. At first it works out well but a friend of hers finds out and warns her to stop. She doesn't listen to him but keeps sneaking out anyway and finally discovers how dangerous it is. It reminded me of Robin Hood quite a bit but it was more of a hard look at what being Robin Hood might actually be like.

The hero she dresses as is specific to the plot of the book. I don't remember the name but the book has a single word title that is the name of the hero. I think it starts with a K.
I read it at least 5 or 6 years ago.

[Bec]

Possibly Jackaroo? I haven't read it, so I can't vouch for it 100%, but the synopsis sounds appropriate.

[Serena]

Yes that is it! Thanks so much.

[Lobstergirl, au gratin]

Requested three times which means Ann has to read it now.