Neechan
Her name is Reiko, but she's just known as "Neechan" (a term of endearment for an older sister). Neechan has a very strong temper and likes to be direct. That's why she bosses Kippei around and usually resorts to insults/threats/violence when he's either stubborn or dense (which is often).
She apparently used to look a lot like her Yuzuyu when she was younger, since Kippei automatically thinks that his sister has somehow shrunk the first time he meets Yuzuyu (either she *really* looked like Yuzuyu when she was younger or Kippei's tragically short on brain cells... I guess it must be a combination of both).