Call me later - Ao no Exorcist OST

Call me later by Mika Kobayashi. From the Ao no exorcist OST

Pic from Zerochan.

Edit: Some people were saying they didn't remember this in the anime. Some times its used:
Episode 4 (When Shiemi is able to walk again.) This one is actually instrumental.
Episode 12 (when Shura reveals herself.)
Episode 19 (When Yukio is telling Shiemi of their mother.)

Just an FYI :3