The The Devil Is a Part-Timer!, Vol. 6 (light novel) — Satoshi Wagahara

The fast-food joint that the Devil King calls his workplace has reopened, now with a hip café space upstairs--the perfect chance for the overlord of all demons to earn a few new certifications and work his way up to management. Meanwhile, Chiho, whose love for the Devil King remains doggedly unrequited, discusses ways with Emi to learn how to master the Idea Link, a skill that could let her telepathically communicate across worlds and call for help whenever she makes contact with a nefarious demon...or angel, for that matter. Suzuno steps up to serve as her teacher...but her choice of training areas (a public bath) leaves the others to question her sanity. It's the sixth volume of this low-wage high fantasy, where soap bubbles fly and everyone may come out less than squeaky clean!