Ask most people in Japan to picture a tonkatsu sandwich and they picture the rosu version. Rosu is the pork loin, cut with its rim of back fat left attached, and that strip of fat is the entire point. Where the lean tenderloin reading is gentle and textural, the loin cut is the flavourful one: the fat renders in the fryer, bastes the meat from its own edge, and leaves the cutlet richer, juicier, and far more assertive between the bread. It is the cutlet sando at full volume, and for a lot of eaters it is simply what the sandwich is supposed to taste like.
The fat cap is also what makes the loin cut harder to get right than it looks. That rim has to render properly, which means the fry cannot be rushed; under-rendered loin fat is rubbery and unpleasant and turns a bite cold and waxy. Done well the fat goes soft and almost sweet, the lean stays moist, and the coarse panko shell shatters around a cutlet that is genuinely succulent rather than merely cooked. The bread is the usual thick shokupan, soft and tight-crumbed, buttered on the inner faces so the crumb does not surrender to the warm meat. Tonkatsu sauce earns its keep here more than anywhere: the loin is rich, and the sweet-tangy sauce with its faint spice cuts straight through that richness, which is exactly why the pairing became standard. A clean cut through a good one shows a defined band of softened fat along the top of the meat, crisp breading, no oil weeping onto the board. A poor one shows pale tight breading, a fat cap still firm and chewy, and a heaviness that the sauce cannot rescue.
What you get is fuller and rounder than the lean cut in every dimension: more pork flavour, more juice, more weight, and a longer finish. It is not subtle and is not trying to be. It is the most flavour-forward of the everyday cuts and the one the whole family is usually compared against.
Within the family the contrasts are sharp. Lean hire tenderloin is the direct opposite, soft and mild with no fat to render. Kurobuta Berkshire and Okinawan agu take the richness further still through breed rather than cut, with their own sweetness and marbling. The double stack, the shredded-cabbage build, the Maisen and Wako restaurant styles, and the soft-roll tonkatsu pan are structural rather than cut variations, and each one deserves its own article rather than being crowded in here.