Probably the most prominent would be the leading Tomb King. He would be in charge of leading his undead forces, making decisions about battles and the defense of their tombs. There might also be necromancers who serve the Tomb Kings, playing important roles in raising more undead and maintaining the magic that keeps the Tomb Kings' power intact.
In the Tomb Kings short story, aside from the Tomb Kings, there could be characters like tomb guardians. These are powerful constructs or undead beings whose sole purpose is to protect the tombs. Another possible main character could be an adventurer who stumbles upon the Tomb Kings' domain. This adventurer's interaction with the Tomb Kings and their world could drive the plot forward, as they might try to steal some of the valuable artifacts from the tombs or learn the secrets of the undead rulers.