Does the word "piece" refers to "bond" or "attachment"?
"Piece" here does not mean "bond" or "attachment." It is actually part of the idiomatic expression to "put a piece of oneself into something/someone," which can mean one of two things: to sacrifice one's time and effort to help others, or to put one's characteristics or personality into someone or something. Based on this definition, we can understand Emma Watson to mean that Louisa May Alcott, the writer of "Little Women," put a lot of her personal beliefs regarding feminism into the characters of the sisters in her book. Ex: I tried to put a little piece of myself in the character I was playing. Ex: Louisa May Alcott put a little bit of herself into the characters of "Little Women." Additionally, a similar expression is to "give a piece of one's heart to something/someone," which means to give someone or something love, affection, or soul. Ex: I gave him a little piece of my heart. Ex: I tried to be vulnerable when I was writing this book. That's why I feel like I give every reader a little piece of my heart when they read it.