Traveling Through a World of Anime Crossover

An idea cut from another story.

Qingshui Ren has lived much, much longer than other people, and has seen far too many things along the way.

After Kibutsuji Muzan died, everyone else returned to their ordinary lives. They rarely gathered together, and transmigrating between worlds so often left him feeling dazed. Back in reality, in the Demon Slayer world, the people around him kept passing away. Eventually, the people he knew were all gone, and he truly set out on a journey with no return.

His family was gone. He was just a traveler from another land, with no place to belong in any world.

Along the way, he met a strange medicine seller, a mushishi, and a mysterious witch.

He saw a girl living among a group of great yokai, though later it was a young man with similar features. The original girl had already passed away.

Along the way, he saw ability users guarding cities. In Ikebukuro, he saw the headless rider searching for the past. He also saw that eternal crimson flame fall. After traveling through a long stretch of time, he reached the era in which he was originally supposed to exist. On the streets of Tokyo, he saw someone who looked exactly like him.

Though they had the same face, the other person's smile was bold and unrestrained, unlike his own, worn smooth by time. The other had many people by his side, while the years had left him old and weighed down by gloom.

--This was the last person in the world connected to him.

When this era, too, had passed, he would truly belong to no world, just as the System had once said.

Those chosen by them were lucky, and also the most unfortunate. They had long years in which to witness life, death, and the mortal world. Time would take everything from them, until all that remained was one person cut off from the world.

They would stand guard over worlds on the brink of collapse.

Theme: It was precisely because he had seen so much love, hate, life, and death, heard so many stories, met all kinds of people, and experienced so much warmth that he chose to protect the world's many different forms.