Should I know who Tsutsukawa Azusa is as of the end of Durarara!! ×2 Ten?
She is one of Karisawa's cosplayer friends, the girl that made Anri cosplay in one of the episodes.



A screenshot of Azurin and Reh-chan.


  Azurin and Reh-chan were amongst Karisawa’s fellow cosplayers, and were about the same age as Anri.


A quote from Volume 10 Chapter 2.