When Dap sends his report to the military bigwigs about Bonzo’s plan to attack Ender, Pace comes to question Graff. Pace is the head of the International Fleet’s Military Police, so he wants to make sure everything is legal at the Battle School and that Graff isn’t hosting a kid-on-kid death match just for fun. (Graff has very serious reasons for holding his kid-on-kid death match.) Apparently, Pace can’t make Graff do anything, so Ender kills Bonzo. Pace also rides the shuttle back to Earth with Graff, Ender, and Bonzo’s corpse.
So why is Pace in the book? To remind us that Graff’s plan is a little controversial (which will come up again, later, when Graff gets put on trial).