Yuusha, Yamemasu Episode - 6

Leo cannot die; his body adapts to any damage or environmental threat.

For three millennia, Leo has watched civilizations rise and fall. He has served countless kings and saved humanity hundreds of times, only to be feared and eventually exiled every single time once the threat was gone. Yuusha, Yamemasu Episode 6

Leo was one of twelve "Demonheart" units created by humanity to fight off an invasion of "Demons" (which were actually extra-dimensional monsters). While the other units were eventually destroyed or deactivated, Leo's unique ability to "auto-evolve" and his immortality allowed him to survive indefinitely. Leo cannot die; his body adapts to any

What the current world calls "magic" is actually a remnant of hyper-advanced nanotechnology and science from the 21st century. Leo cannot die