Beetee, a wounded victor from the Quarter Quell, is a whiz with technology and weapons. The rebels rely on him almost as much as they do on Katniss. Katniss is be the face of the rebellion, but Beetee is the mechanic that makes it tick. He's the mastermind behind all kinds of technical advances, including the creation of new (extraordinarily lethal) weapons and the intercessions with the Capitol TV broadcasts. Without Beetee, the rebels' messages simply wouldn't get out. He can't participate in the fighting – he's wheelchair-bound – but Beetee is always part of the fighting through the weapons he creates.
Beetee was never a warrior, anyway, though. At the end of the book, Beetee votes against vengeance and for "survival" by declining a final and symbolic Hunger Games. He argues, "No. […] It would set a bad precedent. We have to stop viewing one another as enemies. At this point, unity is essential for our survival. No" (26.60). His reasons for voting this way might not be as impassioned as Peeta's, or even Annie's, but they are practical and life-focused. He evades the question of vengeance or justice by going ahead and concentrating on the future. What's more, he's one of the very few Games survivors who get a chance to do just that.