Kuroba Chikage is, to refresh your memory, Kaito's mother. She left a note that said something only Corbeau would know. This makes her a possible candidate.
This, however, is impossible. As you may remember, Kaito saw Corbeau shortly when he was flying back from his encounter with Jii in the first episode. That same evening, he talked to Chikage via Skype (or something like that) and it showed that the sun was shining, meaning she was in a different continent where it was day. No matter how you look at it, Chikage can't be Corbeau. She might've helped him, but I'll come back to that in the chapter: Then Chikage didn't know.
Conclusion: it isn't Chikage.