Kremydi (Κρεμμύδι) is onion, raw or grilled, and it sits in this catalog as a component rather than a dish in its own right. Calling it a sandwich would be dishonest. What it actually is, in the Greek street-food context, is one of the small set of fillings that go into a pita alongside grilled meat, tzatziki, tomato, and a few others. It earns a record because its handling is not incidental: the difference between a sharp, well-built wrap and a flat one often comes down to how the onion was cut and whether it went in raw or hit the grill first.
There are two ways it shows up, and they do different jobs. Raw onion, sliced thin, brings bite, crunch, and a clean acidic lift that cuts through fat from grilled pork or sausage. The work here is in the knife: sliced too thick it dominates and turns acrid, sliced thin and against the grain it stays bright without taking over. Some cooks rinse or briefly soak the slices to pull the harshest edge off while keeping the crunch, which is the mark of someone paying attention. Grilled onion does the opposite job. Cut into wedges or thick rings and cooked over the same heat as the meat, it goes soft and sweet, the sulfur burned off, the edges caramelized. It adds body and a low background sweetness rather than a sharp top note. Sloppy execution on the grilled side means onion that is scorched black on the outside and raw and squeaky in the middle, which contributes nothing.
In practice, which form you get depends on the wrap and the cook. A pork gyros or a loukaniko in pita will usually carry raw onion because the meat is rich and wants the contrast; a leaner build might lean on grilled onion for sweetness instead. Some stalls offer both, or layer thin raw slices for crunch with a few grilled wedges for depth. The full grilled-meat-in-pita format that onion lives inside is a large subject and deserves its own article rather than being crowded in here. The honest summary is short: this is a supporting ingredient whose only job is to be cut right and cooked, or not cooked, on purpose.