Agu pork comes from a small heritage breed native to Okinawa, and an agu katsu sando is the cutlet sandwich pushed toward its most intense and most fatty. The breed is prized for an unusually high fat content and a deep, almost umami-rich flavour, with fat that carries far more taste than ordinary pork fat does. Built into a sando the result is bold and luxurious: a cutlet that eats rich and round, with a savoury depth that fills the mouth and a sweetness in the fat itself. It is a regional speciality treatment, and it is unmistakably the most decadent of the breed-driven cutlet sandos.
That richness is the whole appeal and also the whole risk. Because agu is so fatty, the fry has to render that fat thoroughly or the cutlet turns heavy and slick; under-rendered agu is the failure mode that sinks the sandwich. Judged well, the fat melts down to something soft and deeply flavoured, the coarse panko shell shatters cleanly around it, and the cutlet is succulent without being greasy. The bread carries real responsibility here. The standard thick shokupan, soft and tight-crumbed and buttered on the inner faces, is the necessary counterweight: its plain cool crumb gives the palate somewhere neutral to land between rich bites, and without it the sando would be relentless. Tonkatsu sauce is usually applied with restraint and karashi mustard with a slightly firmer hand than usual, because the breed needs cutting more than it needs sweetening, and a sharp note keeps the fat from coating the whole bite. A clean cut shows generous fat marbled and rendered through the meat, a crisp even shell, and no oil bleeding onto the board. A poor one shows that fat still firm and the sando tipping from rich into greasy.
What sets it apart is intensity. This is the loudest, fattiest, most flavour-saturated cutlet in the family, a sando built around a breed that does not do restraint. Eaten well it is generous and memorable; eaten carelessly it is too much.
Within the family the contrasts are stark. Lean hire tenderloin sits at the opposite pole, mild and light with almost no fat. Rosu loin is flavourful through a single fat cap; kurobuta Berkshire is refined and sweet through fine even marbling rather than through sheer fat volume. The double stack, the cabbage build, the Maisen and Wako house styles, and the soft-roll tonkatsu pan are structural rather than breed variations, and each one deserves its own article rather than being crowded in here.