Story is original and that, I guess, is what keeps people reading this manga. Even when trying to write serious manga (undead MC) it still can be seen as silly shounen story. MC is basically naive pacifist(people trying to kill him so he decides to save them), power levels are all over the place (copper level adventurer can only defeat one skeleton at the time, yet after 10 chapters suddenly all soon-to-be copper level adventurers can defeat tens of them), magic is unexplained and appear when needed as some kind of plot armor(never could use vitality, magic, spirit until he needed to defeat boss). Lastly, rat when becomes MC thrall suddenly gains sentience wants to be dressed as human.
But story is original so we can look past that.
Art is amazing (except swords, they look as if they are drawn by 10 year old child)