James Moriarty is Mayor Sebastian Moran's bastard child
Asides from the obvious canon connection between him and Moran (Moran in the original canon is Moriarty's right-hand man), there were some clues regarding James' identity and role:
- James calls Irene as "sensei" and it's obvious that they already know each other. Irene used to be a former tutor for the Moran children so James calling her "sensei" is not out of the blue. There's also the way he reacted of seeing Irene bleeding after being stabbed by Jack the Ripper.
- James mentions that his parents are busy and don't mind where he is. Moran being a government official is obviously busy and the way he reacted about his daughter's murder gives an idea that he doesn't care about his kids.
- James is said to have a little sister. Mayor Moran's daughter who was murdered is the eldest daughter so it's possible that the girl shown in the opening credits is James' little sister and she really looks like James.
- Mayor Moran is very desperate in retrieving the USB drive which is encrypted. It's possible that it contains records about the Moran children (like birth certificates etc.) and if it fell into the wrong hands, it would expose that the mayor had illicit affairs that would tarnish his reputation. Though she really doesn't know what's in it, Irene probably has an idea what's inside the USB drive. In fact, after Sherlock has the drive, she's focused on other things knowing that she can trust Sherlock with the drive.
- Mycroft, who is Moran's secretary, sees James and didn't say a word as if he knows him.
- Most of the WMG thoughts confirmed as of "Jack the Ripper".
