Basically, if this game wants to correctly depict the time and place it wants to depict, bucklers should be inescapable, and yet from what I can tell there are none.
In essence, using pretty much any one-handed weapon on foot during this time period without anything in the offhand would've been rare, or even considered an outright bad idea. Therefore, everyone who carried a sword, a mace, an axe or anything like that probably should have at the very least a buckler too.
For example, there is no fencing manual documenting the use of an arming sword without a buckler or shield of some sort. It most likely just wasn't considered a good idea. The closest thing to an exception would be some works about the Messer, but here we have to assume either that it was either covering a duel demanding identical weaponry, or some other rather specific context.
Messer and buckler is also covered in fencing manuals and was probably considered more far more effective than just having a Messer.
So who would've used a buckler?
Basically anyone with a one-handed weapon and no larger shield. Archers, polearm infantry, people just travelling from point A to point B, and so on. It was extremely popular.
The big advantage of the buckler is that it was convenient. You could carry it on your belt just like a sword without it being any hindrance to whatever else you are doing, and when you need it, you just grab it and you're good to go.
Sword and buckler is cool as hell, and was probably one of the most common weapon sets of the time. So where the hell are all the bucklers?