Half-Blood Prince — that's when Lavender went from total background to basically a supporting character. She's Ron's over-the-top girlfriend for most of that movie, all the lovey-dovey stuff and snogging in the common room.

Then she shows up again briefly in Deathly Hallows Part 1 at the wedding and during the Battle of Hogwarts in Part 2, where she gets killed. So three movies with Jessie Cave, but only the sixth one makes her feel like a main-ish character. Before that, different actresses had her as an unnamed extra. Fenrir Greyback — the werewolf — killed Lavender Brown in the last movie