TJ might already have shipped off to Vietnam when the book starts, but that doesn't mean he's not important. We get to know a lot about him through flashbacks and Jamie's memories. Some of the best times of her life are playing army with him. What else can bond siblings like saving each other's lives and strategizing to kill your enemies? That it's pretend is beside the point.
All the G.I. Joes in the world can't prepare you for real combat, though, so when TJ gets home from training, Jamie confides in us:
I could tell he was a changed person, and I knew that whatever he'd gone through in basic training had made him stronger and harder, more ready for war. It make me feel like a kid just to stand beside him. (12.3)
Hmm… is it just us, or is this foreshadowing what's to come? TJ changes a lot just from a couple days of training, so imagine the impact of the real war. Jamie might hardly recognize him when he gets home. That said, we don't learn much about what happens after TJ goes to Vietnam. All we know is that he becomes a POW over there, which is super scary for Jamie's family.
Jamie looks up to her brother, big time. Five years older than her, he's still a young guy himself, so while his sister admires him, he still longs for his dad's approval like we dream about chocolate. Before he ships out, TJ tells his dad, "I need you to act like you're proud of me" (12.14). The Colonel's approval means a lot to his kids, especially when they are vulnerable—he's like Yoda, only less green—and in this way, TJ is no different than Jamie. Unless you count the army gear.