Yes, Tactics Online has different armor levels for each side. It would be a very good idea, and Dexterity could increase the armor you have when attacked from behind. And you could have increased hit chances. It would also provide a nice way of keeping the AP special attacks balanced. I mean, if The Doctor uses his 8 AP doom-attack, he might do better damage, but this enemy has excellent Parrying, so he'll step around back.
There shouldn't be an ability tree, so to speak. It seems too arbitrary and not very original. The Doctor would prefer if the skills had to be bought from trainers, like spells. They wouldn't be nearly as numerous, of course, maybe only six to ten, most of them with a status effect and little to no damage.