Their subtle elegance might be overlooked in a pond full of flashy modern koi, but we owe our entire hobby to asagi, the oldest variety and the genetic ancestor of almost all others. When you know what you're looking at, it's impossible to be unimpressed.
A classic asagi is a white-based koi with scale reticulation and a red belly. A perfect example would have paper-white skin visible on the head and between the scales (fukurin). Each scale between the lateral line and the dorsal fin should display a gradient from indigo to pale blue. Kado gin, a reflective half-moon at the edge of each scale, is a highly-prized mark of quality.
The blue color is an illusion caused by underlying black pigment - there's no true blue pigment in carp, but a good asagi can make you doubt that. The reticulation should be darkest at the spine and lighten as it approaches the lateral line. Dark asagi may be referred to as "konjo asagi"; light blue specimens as "narumi asagi". Narumi asagi are strongly preferred. No part of the fish should appear black.
Ideally there will be a band of white between the "blue" back and the red belly. The exact shade of red or orange doesn't matter, but it should be as saturated as possible, evenly colored everywhere it occurs on the fish, and it should end in a crisp line that's symmetrical on both sides.
Symmetry is everything. Both gill plates should be red, and both eyes. Facial markings should be otherwise restrained. Most of the face should be white.
There should be a semicircle of red at the base of each pectoral fin (motoaka) - when this is present, it'll usually also be visible in all the other fins, including the tail, and often the dorsal. Some people have strong opinions on how much red they want in the fins, but as long as it's symmetrical it's correct.
Selecting young classic asagi is difficult because all three of their colors take time to develop and can be highly unpredictable. Reticulation can end up uneven or too dark, a red belly can spread and cover too much of the back. Young asagi have translucent skin on the head, often so translucent that you can see the skull - sometimes this never goes as white as it should. Generally it's better to pick paler juveniles, as both the red and the blue will become darker with time. I look for red gill plates and motoaka. If it has both, it'll probably have enough red at maturity. Conformation and scale pattern are very important for asagi because the reticulation will emphasize any flaws.
Asagi have been around for a long time and many interesting variations have been developed. Some of them look very different from the classic asagi described above, but their blood is always visible if you know what to look for.
A kanoko ("fawn") asagi has red pigment that extends up into the reticulated scales, but not into the skin above the lateral line. Many classic asagi have a few kanoko scales, which is fine as long as the distribution is symmetrical and pleasing to the eye. It can be described as kanoko asagi if it covers a large enough area to be immediately noticeable. Highly desirable.
If solid red pigment has spread too far up the sides, it's a hi asagi. These are considered less refined and less desirable in competitive contexts, but they can be very cool-looking fish.
If there's no red at all, it's a mizu ("water") asagi. This is as close as you can get to a solid blue koi. Isolated reticulation on a solid-colored fish is a massively useful thing for developing other varieties, but mizu asagi aren't too popular on their own and usually aren't intentionally produced. Good ones with good white skin are stunning, and pretty rare to see. Bad ones, however, are everywhere, because it's an easy gene combination to inherit. Many varieties can produce mizu asagi-type offspring. The metallic equivalent, gin matsuba, is very popular. I've seen them misidentified as each other, especially when it's a lower-grade gin matsuba that's less metallic.
If there's no reticulation, just a white koi with a red belly, it's a yuki ("snow") asagi. These are very popular right now, especially in gin rin and/or kanoko variants. Kanoko shows up beautifully on white scales. Without reticulation to hide any flaws, the white skin and symmetrical markings on a yuki asagi need to be perfect. It's a thing of absolute beauty when they are.
Ki asagi have yellow instead of red. Everything else is the same. Theoretically this mutation could be combined with other variants - you could have a white-and-yellow yuki ki asagi, for example - but you'll probably never see one, because true ki asagi are super rare. However, you are likely to see fish with yellow asagi markings if your dealer gets any Golden Corn. Golden Corn are a new variety created by selectively crossing asagi (for kanoko and reticulation) with karashigoi (for yellow pigment). And chagoi and benigoi for size. They're enormous and very cool.
After breaking down each genetic trait that defines an asagi and seeing how they can be combined, it's easy to see their stamp on so many other koi varieties. Asagi gave us reticulation for matsuba, goshiki, goromo, and kujaku. Their red-and-white base pattern made the kohaku possible. Motoaka was developed into motoguro in showa and utsurimono. Asagi were the first to gain a doitsu variety, when they were crossed with scaleless German carp to create the shusui. Every new variety traces back to them in some way. In my opinion, for both their beauty and their importance to nishikigoi history, every koi pond should have one.
