User blog comment:Vina T/Tokyo Ghoul and Assassination Classroom Collaboration/@comment-1419424-20150316152921/@comment-174.118.246.126-20150316155701

The yellow text at the bottom of Mukuro Kyuubi

"3月末より実施予定の無料アップデートでジャンプ作品のコラボ衣装が手に--(I think 入る got cut off in the end?) "

Obtain JUMP's collaboration costumes in the free update scheduled at the end of March

So it is part of the update ^^