I'm going to go with Heather Hoodhood being a static character. She and Holling do manage to emotionally connect at the end of the novel, and the reader sees perhaps a glimmer of change in her; however, I think that possible change is more of a reflection of Holling's attitude toward his sister. The entire novel is told in the first person perspective. I think the emotional connection shared between Heather and Holling at the end of the novel results from Holling's dynamic change and not necessarily Heather's change.
Throughout the rest of the novel, readers see Heather as a very time period specific, cliche character. She is a stereotypical, angst filled teenager that is in constant conflict with her parents. Heather always has some snarky remark ready for Holling, and whenever something doesn't go her way, she locks herself into her room and loudly blasts The Monkees. There are moments when she does show kindness to Holling. For example, she helps Holling pull down the embarrassing posters of him in his Ariel costume. But those moments are exceptions to her normal behavior. They are not symbolic of some kind of overall, dynamic attitude shift in Heather. In fact, I believe that Holling sees Heather as static as well. I believe that is why Holling doesn't even feel the need to call her by name until the very end of the novel.