I dont remember is it a manga or manhua.

In the first chapter when mc still kid, he went to city with his mother. Then some monster attacking the city. Mc mother went to fight the monster. The monster defeated, but mc mother went missing.

Mc family has a secret to protect like a demon world.

Mc body has some seal. When he broke the seal, everyone except mc family (+ one female childhood friend) will forget him.

It make mc best friend didnot remember him until mc meet him and fmc after summer holiday in middle school.

He remember mc because fmc remember first.

Maybe it's not what you are looking for, but sounds very similar - Makai Kaeri no Rettou Nouryokusha