So, if I understand correctly, she'll be able to drink Nana's blood at any time without any negative effect if Nana becomes immortal. Anemia won't be a problem anymore. Neat.
No matter how many times I reread this chapter, I always find it to be such a perfect confession from Yuu. One of my absolute favorites scenes in any manga.
I'm summoning you here 4.5 years later, sorry about that, but I find the it quite funny Rin would have never realised it because, let's face it, no minor power could make Rin popular. He's a tracksuit-loving shut-in with a coarse sense of humour. Not a recipe for being popular. As opposite to that, Yuu was popular to begin with, so a minor power would indeed only boost an already existing popularity, which isn't so hard.