So I just figured out that Yui and Ui are two separate people. That would TOTALLY explain why Ui kept saying “Onee-chan”! At first, I’d thought that “Ui” was simply Yui’s other personality, and the visual representation of “Ui” was simply the director demonstrating the absolute contrast between the two. As such, my belief was that K-On!! was a suspenseful drama about a young teenage girl fighting bipolar disorder. I thought when Yui was out with Azunyan and Jun-chan as “Ui”, her mellowness was caused by her taking of anti-depressants. But now that I know they’re completely different people, it brings about an entirely new dimension to the show!