Barbarian Quest - Chapter 286

Reincarnation.Or rather... transmigration.Whatever it is, it wasn't something i ever thought i would be a victim of.But here i was...... a villain in the very game i died playing.But... as though that wasn't enough.I am not even going to make it past the first arc.Scratch that.The very first day in the academy...... that's the day i die.(Earlier chaps will involve info drop, the true story starts from the Academy Arc)

The 7-year-old little crybaby met 11-year-old big brother who ran away from home.
[This man became the protector of redemption in his life]

The 16-year-old little dragon fell in love with a 20-year-old big brother.
[It’s a pity that the big brother didn’t recognize him as the little crybaby]

The 23-year-old single father and 27-year-old chief executive meets again.
[This time the little crybaby and his big brother are finally together]

Fortunately we met through hundreds of millions of people, Thank God, for letting us break through many obstacles and get back together again, falling in love with you and being with you again is the greatest fortune in my life.

- Description from Novelupdates

The world was once at war with 10 alternate worlds. This was known as the Concept War and our world won 60 years ago. All of this was concealed so no normal people are aware of this fact in the current time.

After the death of his grandfather, the high-school student Sayama Mikoto is suddenly summoned by the giant corporation IAI. He learns this world is headed for destruction due to the acceleration of the Minus Concepts. To prevent this, negotiations must be undertaken with the survivors of the other parallel worlds so their 10 concepts can be released.

And so Sayama is caught up in the final battle to clean up after the Concept War which left behind so much enmity.