I'm looking at my character stats and there's one that I can't map to any particular skill in my brain: Deftness. What does deftness do to affect a character?

It's the main stat for Thief and Ranger classes. It's a bit like agility from what I gather but of a more sneaky nature. Along with other stats it'll determine whether you manage to surprise the monsters during an encounter and get a free round of turns. It may also contribute to the attack order in the first round and possibly subsequent rounds too. It probably has an influence on damage dealt using daggers and bows too as well as determining the success of certain class skills, like stealing.

Booklet: Deftness governs a character's ability to make preemptive strikes, perform critical attacks, and flee from battle.

Wow, I believe I just got served. On that note, I finally got un-lazy enough to sit down and RTFM. By not doing reading I had missed the explanation of Deftness, AND I also missed that Patty allows you to create new characters for your party! That would have made the boss battles easier!
 
New personal rule: RTFM for every RPG, no matter how boring. 
 
Thanks SteamPunkJin