In fact it is MI9... Sorry, don't hit me...
I don't wanna believe that mrs.Kingston-theory; I have own theory..(but it might be possible that someone else has already made it up, but this still popped to my mind...)
I think Sera has been put to the witness protection program three years ago 'cause her brother(Akai) is part of BO and if something happens she could be in great danger(Akai's idea).
(Black could have met Sera then and 'cause of Akai's request he didn't mentioned anything else to Jodie and Camel.)
'Cause of the witness protection program Sera moved to USA to ...(is Sera her first or last name???) Japanese family which has some connections with FBI(propably) and changed her name.
And now after three years she
1. wants to know more about the organization which separated her away from her brother.
2. wants to find her brother
3. investigates other weird things in Japan (Shinichi-is-Conan thing for example)