In a lot of ways, Heather Hoodhood is portrayed as a caricature of a typical teenage girl. Additionally, she seems to function as a way to further highlight the time period for the reader. For example, her flower child persona and specific references to music groups like "The Monkeys" help firmly root the story in its setting.
The Wednesday Wars is told from the perspective of Holling, so it's tough to get a straight read on other characters. The reader's impressions of people are definitely skewed, because we are looking through the tinted lenses that Holling is wearing. For that reason, I would describe Heather as a typical, angst filled teenager that is trying to find ways to rebel against her oppressive and distant parents. I feel this way, because Holling himself feels distant from his sister. It should be noted that Holling doesn't even refer to her by name until late in the book.
Holling and his sister do not appear to get along, and the reader gets the impression that she is quite self centered and focused on herself; however, as the story progresses, the reader learns that the initial impression of Heather is wrong. While it appears that Heather could care less about Holling, that is incorrect. If she truly didn't care about her brother, she would not have helped Holling pull all of the embarrassing pictures of himself down from school. It may not appear that Heather is a caring and loving sister, but her actions (when it really matters) prove otherwise.