Squid Girl

Squid Girl, also known as ''Shinryaku! Ika Musume (侵略!イカ娘 Invade! Squid Girl'') is a Japanese comic series by artist Masahiro Anbe about a young squid girl that emerges from the ocean in order to invade Earth to punish them for polluting the ocean, but ends up working at a shop nearby in order to work off a debt. The series has also been adapted to animation.

Squid Girl outfit announcement
On April 1st, Masahiro posted a drawing of an Inkling wearing a School Uniform to his Twitter account. This was followed in May with Squid Girl and Splatoon crossover artworks by Masahiro, including artworks in the comic magazine, Weekly Shounen Champion.

A page in the Weekly Shounen Champion magazine announced an upcoming Splatoon release:

"そしてついに「侵略！イカ娘」コラボコスチューム制作開始！　無料アップデートでオリジナルコスチュームの配信が決定！　果たしてどんなビジュアルに！？配信開始予定の８月を待て！"

- Weekly Shounen Champion

In English, this translates as:

"And finally, the "Squid Girl" collaboration costume production has begun!

It has been determined this original costume will be delivered as a free update! What is it expected to look like? Wait for the planned distribution to start in August!"

- Weekly Shounen Champion (Translation)