In gamplay terms, Kimahri or Lulu. Neither of them offer much to the party later on in the game, especially once all their abilities begin to cross over and the only thing distinguishing them is their Overdrive.

In character terms? It's gotta be tied for Tidus and Lulu, easily.

And a little sympathy note for all those who say they like Seymour: He's a fantastic villain, one you can really dislike. Not many are that well crafted... except Seto Kaiba, but he's not actually a villain anyway so I suppose he doesn't count.