In its original inception in Genealogy of the Holy War, the Duke Knight class is introduced as the promoted form of the Lance Knight class, wielding Lances as its sole weapon of choice.
In Thracia 776, the Duke Knight class, like all other mounted classes, is forced to dismount from its steed in indoor chapters. When dismounted, Duke Knights lose the ability to wield Lances and are locked to wielding Swords instead.
In Genealogy of the Holy War, Duke Knights have much higher Strength than Paladins, but their Skill and Speed are slightly lower. They can attain an A rank in Lance proficiency.
In Thracia 776, Duke Knights are outclassed by Paladins in every way because of a new weapon rank system, both classes having the same stat caps, and the dismounting feature makes them both switch exclusively to swords dismounted.