Isn't this a cosplay? I think I remember seeing another character wearing a similar outfit but can't find it now..

Isn't Sonico wearing something like this?

Chiera said:

Isn't this a cosplay? I think I remember seeing another character wearing a similar outfit but can't find it now..

It looks pretty similar to Mochizuki Chiyome's first accession clothes in F/GO.

It mentions Assassin Paraiso in the text and commentary so yes, it's a cosplay.