Lulu's the default tosser on my game. Yuna comes in next then Rikku. But I tried killing Yuna and Lulu off to see Rikku toss the ball.

Actually, the affection rating also depends on who use items on most of the time. Also the Guard and Sentinel abilities are crucial to the affection rating. Have Yuna (or whatever girl you want to toss the ball) cast provoke on the enemy (if they have the skill) and have Tidus cast Sentinel in turn!