Honestly, the cast is huge but a few stand out as the absolute core. Obviously, it's all about Vandalieu, the death mage himself. Watching him grow from a reincarnated, persecuted baby into this terrifyingly powerful and oddly wholesome ruler of a monster nation is the whole hook. His companions are basically his found family, but some have more narrative weight. The vampire girl Eleanora feels like his first real 'party member,' and she evolves a ton.
Then you've got the tragic ones, like the ghouls Bellmond and Basdia, who get deeply explored backstories. The side characters from the evil cult and the heroes who reincarnated with him, like the 'Braver' Heinz, are crucial as antagonists, showing the dark side of a traditional hero party. It's a massive ensemble, but the story does a good job making you care about the fates of the monsters more than the humans.