Cool, so I learned how to fight, and crunched the numbers. You should basically never put be forward for battle when you can.

I've compared the stats we have for the heroes and my Findings are so:

Vs Ashe: Win 2 / Lose 3 / Draw 0
Vs Cloud: Win 2 / Lose 2 / Draw 1 (If I use my abilty, it's Win 1 / Draw 2)
Vs Firiton: Win 2 / Lose 2 / Draw 1 (If I use my abilty, it's Win 1 / Draw 2)
Vs Laguna: Win 2 / Lose 2 / Draw 1
Vs Rinoa: Win 2 / Lose 3 / Draw 0 (stat dependant)
Vs Yuna: Win 2 / Lose 2 / Draw 1
Vs Zack: Win 2 / Lose 2 / Draw 1 (If I use my abilty, it's Win 1 / Draw 2)

It's balanced well in most cases. I lose against Ashe and Riona straight off and draw against the other 5, but if I use my ability I drop to lose 5 / Draw 2.

Until I can buff stats, or I just sack my ability entirely it's best to leave me on the bench.