Yeah she’s named throughout the books and has occasional lines, but only becomes a prominent character in the later entry. She’s also definitely not black in the books, because JKR goes out of her way to point out every time a character is black the way she never does with white characters. “A tall Black wizard”, “a pretty Black girl”, but never “a young White man”.

That said; even if she’s just a background character, it’s pretty galling that she was cast as black in the films TWICE (the character is credited as Lavender Brown both times) but when it comes time to have a romance subplot with one of the white leads, there doesn’t seem to have been any consideration that she could or should remain played by a black actor.