In earlier Fire Emblem games, there was no Knife-type classification for weapons, so
anything resembling a knife would have been classified as a Sword. In later ones, however,
additional weapon types were introduced, falling outside of the
Weapon Triangle, allowing characters who
use them to avoid having to worry about the triangle. That being said, Arpeggio has an optional
Secondary Weapon Triangle that includes
Knives, in which they are good against Bows but bad against Whips.
To simulate the tendency of Knives in Fire Emblem to be weaker weapons wielded by faster characters, Knives in Arpeggio tend to deal less damage than other types of melee weapons, but usually automatically get two hits per turn. This applies not only to their basic attack, but to any special attacks executed with them; for special attacks which themselves strike multiple times, this amounts to one more hit than usual, as opposed to the total number of hits doubling or something like that.