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.)