Meh, I was going by the Sohei.
Hmm...synonyms for Mettle that I like include (but are not limited to): ardor, dauntlessness, grit, guts, indomitability, moxie, pluck, resolution, resolve, and vigor.
So what should Mettle be renamed? Hmm...ardor, grit, and vigor are winning for me right now.
(I don't want to use the Hex/Crusader version of Mettle for two reasons; one, to keep those classes distinct; and two, because Tough Defense becomes less useful if the Fighter gets an Evasion-like ability that can be applied to both Fortitude and Will aready)