Scenario Writer Kazushige Nojima offered his insight on this:
OPM Magazine, Issue #75, December 2003, Page 128
"Yuna is more active and beginning to show her feelings more openly, and Rikku has been like that from the beginning. With the two being a lot more talkitive, there had to be someone with a calm, cool demeanor who could put a halt to their execessive chattines. If it were Lulu in that role, she would have completely shut them down, and the story wouldn't have advanced at all. But in Paine's case, she she's someone who can remain neutral, keep the others focused, and help the story carry on together with these two girls."