I know she wasn’t a major character in the book either, but she should have been in the train compartment during the dementor attack. The get-well card she gives Harry when he is in the hospital wing would have been a nice touch too:

Hagrid sent him a bunch of earwiggy flowers that looked like yellow cabbages, and Ginny Weasley, blushing furiously, turned up with a get-well card she had made herself, which sang shrilly unless Harry kept it shut under his bowl of fruit.