Tag is only to be used when the protagonist of the story was a former hero. Tag can be used even if the protagonist goes back to being a hero later in the series. However, the protagonist should not be a hero at the start.
The strongest hero of humanity, Kim Sung Bin.
He threw everything on the line and fought but... He still couldn't prevent the destruction of the world.
But his legend has just begun as he returns 20 years into the past.