Ignoring the games I haven't played enough of (or recently enough) to accurately give responses to...
3. Umaro from Final Fantasy VI - Sure he's useless if by 'use' you do not include 'awesome fun, easy to use, will destroy everything'
4. Gau from Final Fantasy VI - Gau is insanely overpowered. I would use cheap random skills I got and even during endgame they would kick all butts. I never went out of my way to look up how to get things, he just got 'em and I used them and realised how epic those things were. My most commonly used rage was the one where he would do that stone throw at everyone and muddle them. It got me through battles at the end which I should not have otherwise got through.
5. Relm Arrowny from Final Fantasy VI - Ultros may disagree.
6. Gogo from Final Fantasy VI - 6. Article Writer from Buzzfeed - What now!?
8. Yuffie Kisaragi from Final Fantasy VII - Okay this I can kind of agree with, but I don't mind her.
9. Cait Sith from Final Fantasy VII - Plot relevance, otherwise, eh.
10. Selphie Tilmitt from Final Fantasy VIII - Nah, Selphie is boss for those who know the trick Jess mentioned. One-shotting the most powerful enemy in the game, total hax.
11. Princess Garnet Til Alexandros XVII from Final Fantasy IX - She's not useless for those who want a healer that isn't an annoying little prat.
12. Kimahri Ronso from Final Fantasy X - stfu Kimahri is cool.