For example, knights have S compatibility with swords, A with spears, B with daggers, D with katanas, and E with knuckles. I understand from a flavor standpoint that this represents how skilled that class would be with each weapon, but what mechanical impact does this have? It doesn't prevent you from equipping weapons with low compatibility.
My knight does significantly more damage right now with the katana I have on hand than the sword, though, despite the big difference in compatibility. What is the advantage of using more compatible equipment? Why bother paying attention to this?
Best Answer
Compatibility strictly affects the stats that the item gives. The only stats modified by compatibility are P.Atk, P.Def, and M.Def. At rank E, equipping an item gives you the base stats for that item. With better item compatibility, item bonuses get boosted by 20% for each rank. So, at S-rank compatibility, you actually boost item stats by 100%.
Right now, judging by your situation, I believe you just unlocked the Swordmaster class or are getting close to do so. At this point in the game, good swords are a little scarce and you're being given katanas as those are the weapon specialty of the Swordmaster class.
As a side note, you should take into account the special attacks of each weapon when choosing an appropriate weapon for a class. For example, Axes often have the highest P.Atk stat of any equivalent weapon in the tier. However, the trade-off is that their special attack is activated by defeating enemies, making it difficult to unleash multiple special attacks in one boss fight. On the other hand, katana special attacks are unlocked through Defaulting which is good for certain classes, yet in-optimal for others (such as Ninjas, who spend more time attacking than building up BP through Defaulting).