Dink is a minor character who plays a large role in the development of the story. He is the fourth of Ender's true friends, preluded by Petra Arkanian, from Salamander Army, and of course Ender's Launchie friends Shen and Alai.
Where Petra was the person who taught Ender to shoot. Dink was the person who taught Ender to survive in and out of the battle room. Dink was Ender's first toon leader, it is during this time when Ender excelled in the battle school. Dink taught Ender everything that he knew about the game and they fed off eachother's brilliance.
Ender would bring what he learned from Dink Meeker to the Launchie "Free Play" practices that Ender was conducting. It is here that Ender learns how to command his own group.
Dink also influences Ender in other ways. He was the first to suggest to Ender that the true enemies weren't the other students, instead the true enemy was the teachers at battle school. That it was the teachers who were despicable.
Dink believes that the teachers have so much power that they were literally driving the students insane. He cites the example of Bonzo Madrid, who was obsessed with his Spanish Honor so much so that "being better than him is an insult, to be stronger than him, is like cutting off his balls.".
When Ender asks if Dink is crazy he responds:
"I be crazy too little buddy, but at least when I be craziest, I be floating all alone in space and the crazy she float out of me. She soak into the walls and she don't come out til there be battles. And little boys bump into the walls and squish out the crazy."
Dink continues commanding Rat Army after Rose de Nose graduates. Partly because Ender convinces him to take command of an army. Something that he had refused up to this point due to the fear of becoming crazy like the other commanders.
Dink eventually becomes one of Ender's most reliable commanders in command school. There are also hints at a romantic relationship between him and Petra Arkanian, but it can also be explained as being very close friends. As the only real evidence is the pair of them spending time together and holding hands on one noted occasion.