Affiliate Marketing Secrets

I remember reading a comic book in middle school about a young girl who knows about the secret world of monsters. I remember the girl was mature and very serious with short hair. I only remember two parts from the series: one was when she was babysitting a neighbor’s baby, only to be snached away and sold into the monters slave market. When she attempts to save the baby, she ends up being part of the slave auction. She was saved when a ally from the monster’s realm buys her. In the end he tells her that the baby could not be saved because he was already bought. To cover up that the baby was missing they use a goblin (I think) to disguise as the baby for a short time. The second one I remember was when she unexpectly gains popularity with her classmates. She then finds out that she has a doppelganger that is slowly taking over her life. She or the doppelganger confront the other. The doppelganger then begins to tell the girl all of the achivement it has done in short amount of time it was inpersonating her. About how it was more popular with her family and classmates, how it did better in sports and academically. I believe that the girl was beginning to disappear because she be believed the doppelganger was better at being her, until it said to her that it made a better her then she did. The girl had snapped out of it and begain to tell it that it can never be a better her then she was. Thus killing the doppelganger into a smokey shadow. If you know any mangas or comic book story that is similar to what I described, feel free to suggest it.