Naruto: Butterfly Sage

After transmigrating into a branch family of the Akimichi clan, Akimichi Zhengyang only wants to live until his next meal.

But then war comes first.

He doesn't want to be a human shield on the battlefield, so he spends three years modifying the clan's forbidden drug into Everlasting Butterfly. His butterfly transformation never ends, and his chakra burns around the clock. Others rely on talent; he relies on eating. Tsunade says he won't live past twenty-five. He says that's long enough to finish the war.

From the Land of Grass to Kikyo Mountain, from the misty sea of Kirigakure to Kannabi Bridge, he's always one step away. He has saved others, and watched helplessly as fate swallowed everyone whole.

Later, he goes to Shikkotsu Forest. The Sage says, "There's an ancient bloodline within you, and it's fading away."

Zhengyang asks, "What should I do?"

The Sage says, "Eat first."

So he sits down in the forest of bones and, one bite at a time, slowly eats himself back from a weapon into a person.