Saihate no Paladin

A man dies alone after living a stagnant and fruitless life—but that's not where his story ends, as a boy named William is born into a land of fantasy and magic. Raised in a city of the dead by his new family—a mummy, a skeleton, and a ghost—he can remember glimpses of his past life, enough to know his time on Earth was wasted, and he vows not to let that happen again. Schooled in magic and combat, Will sets out on a journey to see the world. Along the way, he'll meet beings of all shapes and sizes. Each of these people has a story to tell and a lesson to teach Will about how to live his life to the fullest.

(Source: J-Novel Club)