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 separately.
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 to avoid confusion, the wiki will generally be using "battlesuits" for the playable versions of Valkyries/other characters.
Battlesuits can be equipped with a Weapon and up to three Stigmata.
Each battlesuit has a starting rank, either B, A, or S, and can be ranked up to SSS via fragments/souls. Check out the handy Evolution and Fragment Farming Guide for how to upgrade collected battlesuits.
As they level up, their skills can be upgraded as well via 
Coin and later on also 
Advanced Skill Materials.
