It really sounds like "COO-pee" to me. Maybe the name isn't Welsh. The other characters living in Wales are
Jessica,
John, and
Nico, and those aren't Welsh names either. It might be an unusual nickname that wasn't explained in the film.
http://www.youtube.com/watch?v=8Q2LedzsH4Y (hear the name at 12:30)
http://www.youtube.com/watch?v=HB2rYzsiuxs (hear the name at 00:30 and several times starting at 00:50)