The Kit Kat rolls come in three flavors: uni (sea urchin), maguro (tuna), and tamago (egg). And while that sounds very, very gross, the rolls are just facsimiles and don't actually match their flavors. For example, the bright-red "tuna" roll (pictured above) is actually a raspberry Kit Kat on top of puffed rice. You see! It's all pretend.
The yellow option pictured below, "egg," is a pumpkin pudding-flavored Kit Kat placed atop a puffed rice cluster, covered in white chocolate, and wrapped in real seaweed. And the "uni" sushi is actually just a roll made from a Hokkaido melon and mascarpone cheese-flavored Kit Kat.