[zetsubou-san]

Spoiler for ch 7:

From what I understand, Imagine Breaker does not work on physical attacks, and Railgun, though powered by electricity, is nevertheless a physical attack. Unless of course you consider the possibility that Mikoto was using her powers not only to propel the coin but also to maintain the velocity of the coin while in flight, which, like the iron sand chainsaw, will be negated on touching Touma's hand. (And now I think I've dug myself into a paradox unwittingly )

I too found the Imouto revelations to be uncharacteristic of Biri Biri. Well I guess we'll learn more about this version of Biri Biri as the manga progresses.