As a free extra explanation: the Weapons here become stronger by eating corrupted souls. Those are considered different from those who aren't and it's thought of as the proper way of dealing with them. If a Weapon collect ninety-nine kishin egg souls and one of a witch that way they become a Death Scythe.

I was curious to see of that power-up effect wouldn't also work for a Meister and tried it out. All I got was an upset stomach, though.