As far as monks go, Zell has more in common with Sabin than Tifa. Both are cheerful bruiser types while Tifa has the physicality but not the personality. She's much more reserved and demure than either of them.

Rinoa has very little of Tifa's personality: she's much more impulsive and flighty. I'm guessing the intent is that she matures and grows as the game progresses, so she has to start out flawed and fallible.