Playable characters are characters that can be controlled by the player in 3rd person. They range from permanently available characters like Valkyries or only for a short while like Mechs. In certain events, some limited time playable characters become available.
First Strongest Valkyrja
5th Valkyrja Squad
SP series characters
SP stands for SPecial, therefore these are from Captainverse instead of current HI3 universe and not canon at all.
Limited to Post-Honkai Odyssey open world.