Carn is the commander of Rabbit Army, which Ender is briefly a part of. To put it succinctly, he’s not a huge jerk. He’s actually the model of a gracious loser – when Ender's Dragon Army beats Rabbit, he comes to congratulate Ender and doesn’t hold it against him (partly because he knows Ender is much better). He’s so nice that “Ender mentally added him to his private list of people who also qualified as human beings” (11.87), which, on one hand, is nice for Carn, and on the other, kind of creepy of Ender to say. In any case, Carn’s a good model for Ender and a reminder that some people are OK.