Shinketsu no Kyuuseishu: 0.00000001% wo Hikiate Saikyou e

Subaru is an average guy--a loser, in fact. Becoming a Player who enters mysterious gates and slays creatures to defend humanity is a far-fetched dream. But after several years of constant bullying and abuse, a Door of Trials appears before him, and he finally becomes a Player. But his brother is out to get him, and Subaru is completely outmatched. Furthermore, the Sky Clock is ticking down, which could mean the end of the world when it hits zero. Can Subaru overcome his brother and uncover the truth behind the gates that plague the world?

(Source: Webtoons)