Sera Masumi

Conan, Ran and Sonoko meet someone on a bus, while they're on their way to a hotel restaurant, who's going to the same hotel. On the roof of that Hotel while investigating a murder she says that she is a detective and the her name is Sera.

Her full name is Sera Masumi. She seems to be suspicious of Conan's true identity when she fakes a mistake in her deduction to get Conan to make his.

Sera says that she was born and raised in Tokyo but since three years ago she had been living with her parents in America. But, she just now returned to Tokyo. Also, Conan says he feels like he may have met her before. This is probably linked to the fact that she is Akai Shuichi's younger sister. Since Shuichi also lived in America and works as an F.B.I agent it would make sense that she lived there with him. She was probably notified of his "death" and has come to Japan to investigate. So, also, Conan may recognize her just because she resembles Akai, or he may have actually known her in the past, as she seems to hint that she knew Kudo well.

Sera notices that someone was living at Kudo's house and found out from Mitsuhiko and Genta that it is Subaru.

Sera is seen at her room at the hotel she's living at, deleting photos from her computer, including the one of the reflection of Haibara, and saying "for data at least...its an easy task to accomplish." (This may have something to do with the mysterious girl who appears later with Sera, who looks a lot like herself (Sera) and also like Haibara. That girl is currently still only known as "Sera's little sister from outside the domain" (possibly meaning "half-sister"))

Although, Sera knows the F.B.I was in Japan on investigation (when her brother was "killed") it's not clear if she knows what the investigation was about or who she thinks is responsible for Akai's "death".

Sera seems to definitely suspect Conan of being Kudo. She may even already know for sure.

When Camel shows a picture of Sera to Jodie and James, Jodie doesn't recognize her at first, but then thinks that she resembles someone. James says he doesn't know her, however, he knew right away that she was a girl (unlike everyone else) and had sort of a shocked face when he saw the picture.
He probably remembers her as Akai's sister clearly.

In a later case, Sera is asking Ran and Sonoko about what kind of girl Haibara is, and Ran says that she's like an adult who's become a kid because she's always saying adult like things. Then she says "You may not know this, but just like Shinichi, Conan is also a huge Holmes fan." And, Sera then thinks to herself  "Yeah, I know...I've known since a long time ago." Since she must know Conan's true identity, and likely knows Haibara's now too, at this point she probably has a fair amount of knowledge of the overall situation but not everything.