Someone told me a theory that Brain is just a figment of Brendan's imagination. He noted that you never see him interact with anyone but Brendan and that he just kind of appears out of nowhere in that last scene, only to walk into Brendan's head as Kid A mentioned. I'm not sure if I buy into it, and I also don't see any thematic or subliminal reason why Rian Johnson would even have Brain be a hallucination in the first place. It doesn't change anything that happens in the story either way.
Oh, and P.S.- Brain is probably my favorite character in the entire movie, and I also snicker to myself anytime he says that line about Pin being old like 26. One other personal favorite moment of mine is when Brendan is shaking down Dode and the Piehouse rats come up. He just whips around and prattles off "So who wants to?! I've got all five senses and I slept last night, that puts me six up on the lot of you!"
"You got me, I'm a scout. You've got heart, kid- how soon can you be in Philly? Tough winters, but they've got a good public transit system."
Man, I just realized I could quote this entire movie. And now I want to watch it again. It really was one of the best of last year.