Should I know who Tsutsukawa Azusa is as of the end of Durarara!! ×2 Ten? When Anri dials Mika at the end of the last episode of Durarara!! ×2 Ten (episode 24 of ×2), we see a shot of some of the contacts on her phone.



Entry #6, highlighted in the screenshot above, is someone named Tsutsukawa Azusa (筒川アズサ). As of this point in the story, am I supposed to know who that is? It's awfully difficult to keep track, what with DRRR having so many characters. 

(Incidentally, the rest of that list is, in order: Akabayashi; Kadota; Karisawa; Shinra; Kida; Aoba; (Tsutsukawa); Togusa; Harima. Curiously, she only has 12 contacts in her phone.)
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.