A Cait Sith actually makes a cameo in the Honeybee Inn, if you peer into "The Lovers" room, a miniaturised version is seen to the right, bobbing around.
For me, Cait Sith has always been somewhat of an enigma throughout most of the game, granted, he "forces" his way into the group, but you must think Cloud's fuse must be going thin after the all the betrayals. Just slice through right that toy Mog. Though perhaps he symbolizes the ability for forgiveness? Reeves isn't all bad.
