The manga/novel takes place in a fantasy world set (many) years after the victory of humanity against demons ,dragons,...(and other races with their factions) thanks to 'hero Sid'.

The MC is a student of a military academy .One day when patrolling the tomb/(graveyard?) of demons. He then gets transported to a parallel world were the world is at war and humanity is losing . After saving his friends from a daemon they don't remember him nor the existence of hero Sid (the other races but humanity had a hero each).

This is the premise and more or less the first chapter. (he also fell in the graveyard when he was small and saw the demons trying to grab Sid's sword (and I think a daemon was protecting it from the rest))

After that his friends feel something disturbingly familiar about the MC. He then manages to go back to the tomb

where he meets a hybrid girl that is chained ,after freeing her with the sword of the old hero (and they fight other hybrid-ish things)

From then on it is him trying to convince everyone he is not an enemy and trying to solve the mystery of his existence, the war ...

In the first ark he sneaks his way to the the demon heroine

(through the subway) after an intense fight and almost losing he asks her about hero Sid .After remembering him , the hybrid-ish things appear again. She then shows her true form and fights the hybrid-ish things off but dies. (She tells them to seek help and that she was a friend of Sid) (the first ark doesn't end there)

(I read the next ark on the novel but I think this is enough)

I really liked this manga/novel because the MC isn't OP (that doesn't mean he is weak) (and it showed casualties of war, one wrong command kills a whole platoon ); it also showed division between all the factions (some were in civil war) and the characters were more or less likeable .I kept reading it because there were many clearly defined questions with an answer in sight (the pacing was good and not many cliffhangers ).

Help me find it please. 😢

I'm pretty sure this is it. Naze Boku no Sekai wo Daremo Oboeteinai no ka?

Thanks that was the one . 😀

It is nice to see the fight with Vanessa drawn, yet it was more cheesy than imagined it . I guess the translator (and my imagination) did his part in making it more interesting .

I guess I will catch up to it in this quarantine. 🙂