You're from Britain and had moved to Japan to become a demon slayer. You had made many friends and had defeated demons once and for all! You were all free!

It was Bonfire night and you had a pretty emotional connection to it, so you went outside by yourself, lighting fireworks- and silently wish you're family was there...Zenitsu was outside taking a midnight walk and he saw you, all sad and alone...

(I don't know, decide what he does next)