Doitsu is Japanese for “German,” and the name is literal. In the early 1900s, Emperor Meiji had German mirror carp, a food fish bred in Europe for reduced scaling that made it easier to clean, imported into Japan. Japanese breeders crossed those German carp with existing Nishikigoi bloodlines, and the result was a koi with the same color varieties everyone already knew, Kohaku, Showa, Asagi, and the rest, but with a dramatically different scale pattern.
A true Doitsu koi has one of two looks: fully scaleless, with smooth, leathery skin, or “mirror scale,” with a single row of large, glossy scales running along the lateral line and sometimes the dorsal ridge, while the rest of the body stays bare. Both types show off color and pattern differently than a fully-scaled koi. Without a full coat of scales creating texture and shadow across the body, Doitsu koi tend to look glossier and their patterns can appear bolder and more graphic, almost painted on.
Doitsu isn't its own variety the way Kohaku or Bekko is. It's a scale type that can apply to almost any pattern, which is why you'll see Doitsu Showa, Doitsu Ochiba Shigure, and Doitsu Kumonryu (in fact, Kumonryu is Doitsu by default). For show judging, Doitsu koi are typically judged within their own division rather than head-to-head against fully scaled koi of the same pattern, since the scale difference changes how the pattern reads enough that comparing them directly wouldn't be fair to either.
Doitsu koi tend to have a devoted following among keepers who like a cleaner, more modern look in the pond. Because the mirror scales catch light differently than a full scale coat, a good Doitsu Kohaku or Doitsu Shusui can look strikingly different from its fully-scaled counterpart despite carrying the exact same pattern.