There are japanese koi varieties that have rows of darker scales along the dorsal fin
called Shusui.
What you have pictured looks more like a ghost koi.
Growing up we had many Ghost koi. They are a cross between Mirror Carp and Koi.
one individual is still going at about 28 years old and close to 3 feet. This fish is about 18" deep and has a build like a Carp. Obviously he took more after the carp genes, but many of them stay thinner.
Are you in Europe? The Mirror koi were created there and are cheaply readily available as "Ghost Koi".
They can have any color pattern that koi can, its more common to get a shiny metallic type that takes more after the carp.