He was the summoned hero for Ariadna-goddess' B class world, and he failed to save it, and now he's super cautious and has all of his powers still, and remembers the friends, and how much he was in love with Ariadna, and blah blah fucking blah....
And when he defeats the Demon King, and they finally win, it ends with him becoming some kind of "Break Glass In Case Of Emergency" type of hero/existence, and Ariadna is super sad because she loves him, and the new goddess is all proud and shit.