seniority matters but your availability does as well. If you're able to do his full schedule and he can do yours then take his hours otherwise ask your schedule maker to explain it to you. If they can not explain it to you then you'll want to look into filing a grievance.

The scheduling application that Kroger uses takes into account your seniority when it does an autofill, but only to a point. Beyond that the changes have to be made manually.