The Abandoned Son Who Returned as a Calamity

In his previous life, Kairo was just an ordinary seventeen year old boy who loved anime and games and died alone at his desk without anyone noticing for four days. No family. No one to mourn him. Just a boy, a game controller, and a heart that quietly gave out on a Tuesday.
When a goddess offered him a second chance, he chose carefully - a body that would grow beyond human limits, magic that would evolve beyond what this world calls possible, and a system that would awaken on his seventh birthday and break every scale it tried to use on him.
He opened his eyes as Kairo Ashcroft - the unwanted adopted son of a Duke who declared him worthless before he could walk. Raised in the servants' wing. Compared daily to his favored stepbrother. Written off by everyone in the manor as a mistake with a last name.
Everyone except her.
Seraphina. The woman who walked into an orphanage and chose him when nobody else would. Who carried them both through four years of a Duke's contempt and cold corridors and never once stopped being warm. Who cried behind doors she thought were thick enough and healed his wounds with shaking hands and apologized for things that were never her fault.
He made her one promise when he was six years old.
Three months. Then we're leaving.
He meant every word.
Now Kairo is seven years old and has left the manor behind forever. He has a system that cannot measure what he is, magic that rewrites the rules of what is possible, and weapons he forges himself from materials that should not exist in any market in any kingdom. The guild calls him C rank. The system calls him something the numbers cannot contain. The demon lord territories are already sending things to investigate what woke up on his birthday and shook the world like a warning.
But Kairo has exactly one goal.