When people in Japan name a single katsu sando by brand, the one they reach for first is usually Maisen's. The Tokyo tonkatsu restaurant's sandwich is the reference version of the genre: a deep-fried breaded pork cutlet, sauced, between soft white bread with the crusts cut away and the sandwich trimmed into neat triangles. The thing that sets it apart from a generic katsu sando is the pork itself, fried so tender that it gives almost no resistance to the bite, and a finish so consistent across department-store counters and airport cases that the brand has become shorthand for what the sandwich should taste like.
The craft is in the cutlet and in restraint everywhere else. The pork is a lean, well-trimmed loin or fillet, pounded and handled so the meat stays soft rather than tightening in the fryer, then dredged, egg-washed, and coated in fine panko and fried until the crumb is pale-gold and shatteringly crisp while the inside stays juicy and pale. It is brushed with a tonkatsu sauce, fruity, dark, sweet and tangy, applied to flavor without soaking the breading into mush. The bread is soft shokupan, thin, sometimes lightly buttered on the inside face so the bread takes the sauce's moisture instead of the crust going limp. Crusts come off and the sandwich is cut into clean triangles, both for the look and so each piece is a tidy bite of the same ratio. Done well the eat is a single soft pillow of bread, a thin crisp shell, and pork that pulls apart almost without effort, the sauce a background sweetness rather than the main event. Done poorly the cutlet is dry and chewy, the breading goes soggy and slides off, the sauce overpowers, and the bread tears.
Eating one is meant to be undemanding. The cutlet does all the work and the bread is deliberately plain so the meat reads clearly; the triangular cut and removed crusts make it a clean, portable thing rather than a knife-and-fork plate, which is why it travels so well from the restaurant counter to a bag on a train.
The variations are mostly a matter of grade and cut. A premium fillet (hire) version uses the leanest, most tender cut; a rosu loin version keeps a richer fat line. Some sets come three triangles to a box for a depachika counter, some thicker and rarer for a restaurant plate, some plainer for an airport grab. Other brands and the broader everyday katsu sando run the same idea with a heavier hand. The generic restaurant-counter katsu sando, where the build is the genre rather than one house's signature, deserves its own article rather than being crowded in here.