FourthBear said:I'll note that I do recall a forum post that Mearls noted that you can indeed add class levels onto a monster. Frankly, looking at the Pit Fiend stats, I can't see anything at all to prevent it. Monsters have ability scores, defenses, skills and pretty much everything that a class could add to. If classes are like they were in 3e and primarily additive, you should just be able to add on the class abilities and skills as appropriate. It might be a bit clumsy, as in 3e, but I see nothing that should prevent it.
I hate to harp on about gear, but it does pose an issue with giving monsters PC classes. Gear is factored into PC class abilities. We can see from the Pit Fiend example that gear tends not to be factored into even those monsters which could reasonably use gear (and in fact are described as wearing it?!). So to keep the PC class levels appropriately powered, you either have to give monsters arbitrary bonuses (to make up for the lack of gear) or absurdly weak gear i.e. a Pit Fiend with 2 levels of wizard wearing lvl 2 wizard gear. Giving arbitrary bonuses works, sort of. It is *more powerful* than bonuses from gear though (gear is vulnerable to attack. magical gear is extra-vulnerable through Dispel effects), and poses questions with shape-change magics (In human form, I get a bonus from using a magic sword. In demon form, capable of wielding a sword, no bonus. Huh??). If the Pit Fiend picks up a level of Fighter, he should probably get some armor proficiencies (he might already have some, they aren't listed). If so, he might want to wear armor (neat thought!). Unfortunately, while he *is* described as wearing a breastplate, we don't know the stats of said BP. Which makes making use of the (theoretical) armor proficiencies... hard.