Tidus transformation from a whiny (but sweet and caring) kid into a hero that gives his own life to save Spira is one of the most endearing parts of the story IMHO.

Even his voice changes. Compare his narrating voice (more mature) to his whiny voice at the beginning.

Besides, athletic skill doesn't mean good character. Witness Dennis Rodman or that racist NBA player of recently...

Which is another thing that makes Tidus so endearing to me. He is the "Michael Jordan" of Spira: A talented superstar that does not put on airs and is nice and caring to fans