Quote Originally Posted by Gwelenguchenkus
kInd of funny that they were so apprehensive to have Tidus join the group, then some random al bhed washes up ashore and all of the sudden she's a guardian in the group. What. The. Hell.

I guess that fact that (SPOILER)Yuna and Rikku are cousins helps.
There's also the fact that (SPOILER)Yuna says right away that she wants Rikku to be her Guardian, which she didn't do in regard to Tidus.