She was the girl (or, maybe, the spirit of the girl) who was molested by her father as an infant; when they make Henry recite what he "did", he reveals that the girl's mother drowned both of them in ocean (hence the beach in her story). It's more than likely what Max means when she says, "I'm so sorry what your father did to you."
Come to think of it, all of the girls were somehow spirits(?) of the girls who were hurt at the hands of a man (i.e., the girl who saw her father and brother shot was the girl whose family was killed by a Nazi, the deaf girl had a knife shoved into her).