YUU OKANO

Lento Feyner has been stuck as a bronze rank adventurer for 10 years, but while exploring the labyrinth he accidentally discovers a dragon and is killed. However, he finds himself still conscious in the body of a skeleton, and resolves to kill monsters to take their magical power in the hopes that he will be able to evolve as a monster and obtain a human-like body one day.

When the Demon Tribes led by a Demon King appeared and launched a massive war at mankind, all hope seems to be lost, until a brave group of heroes managed to turn the tide and even defeated the Demon King.John Serieux, a mid-ranked soldier of World's Resistance against the demon's army, had fortunately been able to witness the Demon King's fall by the Heroes, but also was unfortunately killed a moment after. Miraculously, he was still able to open his eyes, only to realize that his mind had been sent back to the past when he's still a baby! With all the memories of his adult years, the knowledge of many scientific breakthroughs during the war, and information of the upcoming great disaster all intact, John is determined to cultivate his potential efficiently and optimally to help change the future for the best.

Eris Boreas Greyrat is a former noble girl, who one day was transported to the other side of the world with her cousin Rudeus. She grew as a swordswoman, but ultimately was unable to save Rudy as he was nearly killed. Deciding that she wants to be someone who can support him rather than the other way around, Eris decides to leave Rudeus and go on her own journey. This is a spinoff of Mushoku Tensei, which splinters off from the main manga at around chapter 51 (volume 6 of the light novel, first season of the anime).

It details the life of a mid-rank soldier who died at the very end of the war against the Demon Race. Fortunately for him, he wakes up to find that he's a baby again, in the embrace of his real mother, years in the past. Determined to avoid the chain of tragedies that took his beloved ones away from him, he takes advantage of the advancements in swordsmanship and magic that developed during the war (as we all know, wars are when many scientific breakthroughs tend to happen), and tries his best to prepare for the demon invasion as well as strengthen himself with maximum efficiency.