Robin is Moon Shadow's only friend his age. Robin is also similar to Moon Shadow in a lot of ways, like how she is away from her birth place of Missouri and she is away from her parents (they've passed away). Robin's family life is also alternative: she and Miss Whitlaw are a family unit. This is similar to how Moon Shadow's family is composed of all men in America. But mostly we intuit that Robin is Moon Shadow's companion because she's always hanging around him, she's not judgmental, and she looks out for him.