Playable characters in Honkai Impact 3rd are separated into battlesuits. A single character can have multiple battlesuits, each of which would have to be unlocked separably.
While the game uses the term "Valkyries" for both playable characters and their battlesuits, not all of them are falling under the lore definition. To try and avoid confusion, the wiki will generally be using "battlesuits" for the playable versions of Valkyries/other characters.
Each battlesuit has a starting rank, either B, A, or S, and each battlesuit can be ranked up to SSS via fragments/souls. Check out the handy Evolution and Fragment Farming Guide for how to upgrade collected battlesuits.