According to the comprehensive BAK WEB website,

"If you've talked to Isaac (see Hawk's Hollow) about Devon, Devon's "POKIIR" keyword will be turned on. This will turn on the "A GAME" keyword. You can now play Pokiir with Devon, or decline. As soon as you decline, a new keyword "SELF-DEFENSE" will be offered ! Devon can raise your Characters' Defense Skill by 10 points (15 points if Selected only), if you agree to his fee of 80 Gold Sovereigns. [Thanks to Sergey Sarnavskiy for helping me realize this.]"

I vaguely remember this being true from playing years ago.

But I've talked to Isaac about Devon and yet the only options I have with Devon are: "The Festival", "Mercenaries", and "Spitzer". Is there some other condition to get the training?

OK, I figured it out. The Pokiir option disappeared when I killed Nago.

This is probably because you found out about Devon as part of that quest, inquiring with Isaac about the mindreaders who are laying out traps for you. So when you've killed Nago, the game considers everything related to that quest closed, even though it makes approximately zero sense in this case.

EDIT: There's no way I can mark this as a solution myself, right? New here...

No... it's set up that way to prevent people spamming questiosn and marking themselves as solution to boost their forum rep. But, thanks for posting back with the solution, it helps others who may have the same question.