Pop Kei (ポップ系) is a colorful Japanese fashion mostly associated with the Decora aesthetic. The term refers to how the brightly colored clothing "pops" in an outfit.
The aesthetic was originally presented in the 90s by designers like Sebastian Masuda and Sayuri Tabuchi, but became popular when the clothing store Spank! opened in 2004. Spank! stores also have very similar styles to Fairy Kei.
This style is mostly based around graphic tees as well as otherwise printed clothing with 80s pop impressions. It is very similar to Fairy Kei but with the main difference that it uses bright colors over pastels. The look is often unisex and can feature trousers, shorts and leggings rather than dresses and skirts. Very popular are thrifted and remade clothes featuring cartoon characters, polka-dots, and denim.
External links to help get a better understanding of this aesthetic.