Since Seymour and Kimahri are refered to as "Seymour Guado" and "Kimahri Ronso" respectively (as well as a bunch of the guado and ronso Blitzball players) Logically everyone elses last names are:

Yuna Human
Wakka Human
Lulu Human
Auron Human
Rikku Human
Tidus Dream (possibly Tidus Aeon)

aye or nay?
genius or nut case?