A hire katsu sando is the lean reading of the pork-cutlet sandwich. Hire is the tenderloin, the long muscle that does almost no work and so carries almost no connective toughness and very little fat. Cut into a cutlet, breaded, and fried, it gives a sando that is soft all the way through, mild in flavour, and noticeably lighter on the palate than its loin-cut sibling. People who find a fatty cutlet heavy between thick bread tend to gravitate here, and people who want the pork itself to be the loudest thing in the sandwich often do not, because lean tenderloin is gentle by nature.
The handling matters more with hire than with almost any other cut, precisely because it has no fat to protect it. Tenderloin overcooks fast and turns dry and cottony, so the cutlet is usually shaped a touch thicker and the fry is judged closely: the panko shell has to set to a clean gold and the meat has to come out just done, still pale and moist at the centre. A good one is yielding under the teeth with almost no resistance, the crust crisp around a tender core. A poor one is the giveaway of careless tenderloin work everywhere, grey and tight in the middle, the lightness of the cut wasted. The shokupan is the usual thick soft milk bread, buttered on the inner faces, and because the pork is so mild the bread and sauce carry proportionally more of the sando here than they do with a richer cut. Tonkatsu sauce is brushed on warm, and many builds lean on a clear stripe of karashi mustard, since the sharp heat has more room to register against unassertive meat.
What you are tasting is contrast managed at low volume. There is no fat cap to render and no marbling to melt, so the pleasure is textural, the clean shatter of the crust against soft bread against soft meat, lifted by sweet-tangy sauce and a thread of mustard. It is the most delicate member of the cutlet-sando family and the one that depends most on a light hand at the fryer.
Set against the rest of the family the position is clear. Rosu is the fatty, more flavourful loin with its rim of fat, the direct counterpoint to this cut. Kurobuta Berkshire and Okinawan agu push richness and breed character in the opposite direction entirely. The double stack, the cabbage layer, the Maisen and Wako house styles, and the soft-roll tonkatsu pan are structural variations rather than cut variations, and each one deserves its own article rather than being crowded in here.