Rinoa is NOT Ultimecia. Even assuming Ultimecia once was Rinoa, to misquote Obi-Wan Kenobi:

"When Rinoa became Ultimecia, the good person who was Rinoa was destroyed."

Rinoa and Ultimecia are two completely different people. Different drives, different emotions, different accents... When someone changes that much, they are no longer the same person, IMHO.