Unheard Wishes: A Tale of Desires - Chapter 31: Payback?

Yuki, a high school junior, dies heroically saving a stranger and is reincarnated in a fantasy world, where he discovers an extraordinary and unrivaled power.

A young man with no spiritual roots. A person who is thought of as trash by others. He constantly acquires various items thought of as waste by the cultivation world. No matter if it is garbage pills or useless materials, he will take them all!

- Description from Novelupdates

Found a suitable body named Bai Xiao. Reincarnating into the World of Elementalists.

“Mission: Overpopulation of carnivorous fish in the lagoon.

Please kill 666 carnivorous fish in order to prevent the destruction of the lagoon’s ecosystem.

Reward: 20 Balance Points.”

“Mission: Forest fire.

Extinguish the fire.

Reward: 200 Balance Points.”

- Description from Novelupdates

I became a mage character from my favorite game.