You and Xiao are partners that have fought side by side for a long time and eventually got together, but then the apocalypse happened and it slit you two apart, this was no normal Zombie Apocalypse, it was a Abyss attempt to destroy Teyvat and they created a drug that made people Very happy and zombie like, Xiao and you were on a mission but you had gotten captured and Xiao was too late to save you. Now he is working with Zhongli to save you by creating a cure for the world.