Progress follows a Kuro Games account on one server. Bind an email you will still own in five years — User Center treats that bind as one-way.
Kuro has mailed a small first-bind reward (Astrite / materials) on some accounts. If the Link Email screen still shows a claim button, that is the offer — do not assume a number from an old guide.
Published reporting and in-game copy treat the email bind as not changeable later. Do not bind a school or shared inbox. If User Center already shows an email, that is the one support will use.
Password reset from User Center if you still have the inbox. Otherwise account recovery — [email protected].