Yu is a punny ghost who is very cheerful. She wants to turn people into ghosts because she cannot understand why people like to live. This is because her life was miserable and tragic and she starved to death becuase of her parents. 
Rei, on the other hand, is very reserved and is more empathetic; but he mostly acts with Yu. When he was still alive, Rei killed himself because his twin sister, Yu died. 
They had extremely abusive and negectful parents when, who indirectly killed them.