With the novel's focus on Garp, it's easy to forget how important Helen is. Over the course of the plot, she transforms from an obsessive over-planner into a mature and strong woman, and as she makes her way, in many ways, she provides Garp with the strength he needs to make it, too.
Remember: Garp only "decided he was going to be a writer" (3.13) in the first place to win Helen's heart. But Helen has a deeper understanding of the way that fiction works than Garp does, so while Garp is a solid writer, his early work is all instinct and no finesse. And Helen is happy to break this reality down for him.
Already, Helen seems destined to become an English professor—her critiques of Garp's work simultaneously teach him and motivate him to work harder. So as much as Garp may be malleable, Helen is the one doing the molding.
This dynamic—of Helen as the one taking the lead, and Garp as meandering except when she guides him—isn't confined to Garp's writing, though, and we can see it play out in their respective approaches to infidelity. On the one hand, there's Garp, who cheats out of a fundamentally selfish need to fulfill his lust. Helen, however, sparks her relationship with Michael only after she feels that- Garp doesn't give her the "attention" (12.126) that she deserves. In other words, Garp follows his instincts, while Helen teaches him lessons.
While Helen is a strong, independent woman, much of her life is structured around helping Garp fulfill his dreams. She supports him through thick and thin, giving him attention when he needs it most and acting a bit like his spine, despite the fact that he refuses to do the same for her. It's only by almost losing Helen that Garp is able to appreciate what he has. Better late than never, we suppose.