Today's Fire Emblem: Three Houses Twitter update has more info than usual! First up, let's get a rundown of Catherine, (voiced by Chie Matsuura).
Catherine belongs to the Knights of Seiros, and is a graduate of the Officer’s Academy at the Garreg Mach Monastery. She’s a skilled swordswoman who uses one of the Raitei, one of the sacred weapons part of the “Legacy of the Heroes.” Catherine has pledged allegiance to Rhea, the archbishop of the Church of Seiros.
Speaking the Legacy of the Heroes weapons, we have some details on those items as well.
Legend has it that a long time ago, Heroes known as “The Ten Heroes of Fodlan” received power from the Goddess herself in the form of sacred weapons, the “Legacy of the Heroes.” This also came with the mysterious crests that we've covered in previous feature. Only someone with a crest which corresponds to a sacred weapon can make full use of the Goddess’ powers hidden within.
The sword used by the protagonist is indeed one of the “Legacy of the Heroes” weapons, and it's called the Heavenly Blade.