The Third Imperial Princess's All-Purpose Butler

Roth is a genius magician who wields the world's strongest defensive magic. His job is to guard Krell, the world's cutest and most adorable little princess. One day, Roth receives a request to protect the “magic key,” a national treasure. The two set out on the job, but it soon develops into a major incident that shakes their and their country's fates... "My butler will come save me, won't he?" The master-servant tale between an ultra-kawaii princess and her sharp-tongued butler is about to begin! (Source: HJ Bunko, translated)