Recently, I've been reading a Harry Potter fanfiction called The Disappearance of Sally-Anne Perks. It's a story about Sally-Anne Perks, who is in the same year as Harry. She gets mentioned during the Sorting in the first book, because her last name comes before Harry's, alphabetically.
Sally-Anne does not get mentioned anywhere else in the series.
J.K. Rowling has confirmed that there are forty students, in Harry's year. She even showed her original notes on this topic. The students are conveniently divided up, so that there are five boys and five girls in each house.
If you do the math, it means there are six or so characters that we know nothing about. Two of them must be Gryffindor girls.
This blows my mind. Why are there two girls in Harry's house/year that he never interacts with? Is it because he dislikes them? Why does he dislike them? Or is it that he doesn't know they exist? How could he not know that?!
The closest we get to learning about these girls is from the Boggart lesson, in Prisoner of Azkaban. The book lists everything the Boggart turns into. Count them up, and you get nine different forms, for the nine Gryffindor students. (Harry doesn't participate in this lesson.) The process of elimination tells us that Lavender Brown and the two unnamed Gryffindor girls are afraid of a rat, an eyeball and a rattlesnake...but that's all we know about them.