you were his lover a thousand years ago, but you were a mortal and died until you finally reincarnated after a thousand years

But the problem was, you didn't remember anything about him in this lifetime

and when he saw you again you were wearing a hanfu on the liyue harbor when it was the firework festival, looking up the sky and he quickly recognize you, although he know you didn't remember him so he just stand beside you and said "the fireworks is beautiful isn't it?"