Of all the onigirazu variants, the umeboshi build is the one that leans hardest on a single ingredient. A pickled plum is intensely sour and salty, almost aggressively so, and inside the flat rice-and-nori frame it works less as a filling than as a seasoning spread across the whole cut face. This is the most traditional flavor in the family, descended directly from the plainest, most enduring rice ball there is, and the format barely changes its character: it just turns the buried sour center into an even seam you taste in every bite.
The frame is identical to its siblings. A square of nori is set out shiny side down, a thin bed of warm rice is pressed onto the middle, the umeboshi is laid in, capped with more rice, and the seaweed corners are folded in so the parcel can settle seam side down before it is cut. The discipline here is about restraint and distribution rather than moisture. Because the plum is so concentrated, it is usually pitted and either torn into pieces or smeared into a rough paste, sometimes loosened with a little of its own brine or chopped shiso, and then streaked thinly and evenly so no single bite is all salt and no other bite is plain rice. Many makers fold the umeboshi through the rice itself instead of leaving it as a discrete layer, which spreads the sourness more reliably and lets the rice carry it. The rice is often left unsalted or barely salted, since the plum supplies more than enough salt on its own. Cut clean, the face shows flecks of dull red shot through pale rice inside a dark seaweed edge, and it eats cool, sharp, and bracing, the plum's acid cutting straight through the starch. Built carelessly, you get a sour bomb in one corner and bland rice everywhere else, or a wet paste that has soaked the nori slack.
What the filling does inside the frame is preservation as much as flavor. Umeboshi is salt and acid, both of which keep rice fresher and safer at room temperature, so this is the version with the longest practical life in a warm lunch box and the one least dependent on refrigeration. That durability, plus the fact that the ingredient list is essentially rice, plum, and seaweed, makes it the most pantry-simple member of the group and the one closest to old-fashioned everyday eating.
Variations stay close to the austere base and add one supporting note: katsuobushi folded in for smoky depth, shiso for herbal lift, a little white sesame, or shirasu whitebait for a savory counterweight to the sourness. Anything that brings in a wet or fried center is a different sandwich and deserves its own article rather than being crowded in here.