I'll say she is related to Eisuke! I've read the latest manga, she sound like him. Have you thought of possibility Eisuke comes to America with chances to get contact with FBI and CBI? What I think is she heard of Akai's death and somehow want to learn more as she believe the cover story is fake. If to find out truth, she can't stay in America, as there's restriction for investigation be allowed to police for better access. In first place she can't join to be police, she has to be detective if to know answer immediately and Japan is the good place for starters. I think the world is small, Eisuke have met Sera as their goal is almost the same, they all keen to join either FBI or CSI if investigating Black Organization or on big mission. They both can speak Japanese so somehow they keep in touch, it's likely Sera meet Eisuke, hearing from him about his visit to Kogoro as his cover story.
I don't think she buy Eisuke's story and suspicious of Conan as she hear from him Kogoro is sleeping to tell his deduction. His story become unbelievable and he didn't give Conan away, I guess it's how she was unenthusiastic to learn Conan is a bright and mature kid. She say in the manga, 'Shinichi has reason not to show up' when she talk to Ran, as if she already talk to Eisuke, i think she did hear from Eisuke about 'Shinichi has reason not to show up.' and she believe Eisuke changing subject by stating facts. (imagine this flashback added to manga)
Sera won't be surprised to hear the name Kudo Shinichi. I feel she was expecting Conan to contact 'Shinichi' and tell his deduction but instead Conan revealed himself as Shinichi through phone. It was on the day she was beside Ran and Conan in the bus, and Sera didn't explain herself properly and misunderstood as a pervert. If she is FBI, it's highly likely she don't want to be reveal as one, rather be misunderstood as pervert. She want to run away but she can't she was in the bus. She was actually following Conan and Ran, with possibility she already know Ran, Conan and Professor Agasa, I haven't see her in previous manga before her appearance, I feel she was researching on them.
I think she is Bourbon. Black Organization say Bourbon is quick and fast in investigation, and Sera is fast enough to be Bourbon. If Bourbon believe Akai is alive, and Conan has see Sera before, it's likely Sera is an imposter as she look like Akai.
If Sera is FBI, she come to Japan with purpose to find Akai as she likely to be related to Akai, and investigating Black Organization as a spy. Bourbon believe Akai is alive and bend to find him. Sera come to Japan to investigate as she want to find out the truth, I feel it is her act of grief, disbelieving Akai is dead.
Bourbon is good at investigating. Sera is also fast. I'll say, Sera's characteristics look similar to Bourbon's style.
This is what I think of Sera.
