This Hero Is Clearly Overpowered, Yet He Likes to Pretend He's Weak

This book is also known as "The One-Shot Hero Loves Pretending to Be Weak"

"The Strongest Hero's Daily Slacking-Off"

"The Whole World Knows I'm Strong, but I'm the Only One Who Thinks I'm Doing a Great Job Pretending"

There wasn't enough room for the synopsis, so I put it at the very beginning of the chapters.

Invincible protagonist, hero and demon lord, gender-bender.