I gave the Invisible Blade a look.
Their own example, straight from the book, has an AC at 11th level that would be low for a 5th level. I mean, you can beat an AC 17 at first level without trying.
The class disallows armor of any kind to get some of its advantages. Specifically, the character gets one point of INT bonus to AC for every IB level.
So their example leads with his face. Against an equal level, well, anything, he'll be dead in two, maybe three rounds.
So I'm thinking that I won't follow their example.
Allowing for similar stats and skill/feat choices, I'd probably forego "Unfettered Defense" class ability and buy some armor. Plain studded leather would match what he has, and Mithral Chain Shirt would double it without penalty. After that, magic it like crazy.
At 11th level, if you can't come up with a 25 AC, stay home.
By the way, this came up because of something that happened in a previous campaign: I was running a straight Scout. One of the players thought it was broken, considering the damage he was able to stack on a single arrow shot.
But the Scout can only do that once per round, because the Skirmish feature requires a Move of 10 feet or more prior to that shot.
I'd argued that a well prepared Rogue with a bow and a good Hide check could exceed it by doing a Shoot/Sneak - Hide combo every round. Rogue sneak attack piles on the dice faster than the Scout's Skirmish damage.
So I've been considering a Halfling Rogue named Oliver Underfoot, a charming fellow with an affinity for the femoral artery.
While Sneak/Hide is a nice combination, DM's get tired of a three foot tall death dealer, and it's too tempting and easy to just shut down the one trick pony. Improved Feint is a decent second trick.
As mentioned though, we had a player who was conveniently unfamiliar with the rules, all while playing the rare combinations to maximum advantage. If my PC even hints at following that player's build I'll be shut down in nothing flat. (And deservedly so.)
So, no matter what books we do or don't allow I expect that the Invisible blade PRC will forever be discouraged/disallowed in our campaigns.
Good tips though. Thank you.