Vikram Singh (played by Sunkrish Bala) is a recurring character from season 8 of Castle.
When Beckett disappears in XY, she appears to be on the run with a man described as "late twenties, East Indian, had [a] beard". Alexis Castle follows up on bits of conversation and trash found at the scene where he was first spotted and comes up with the theory that the man is diabetic, which later turns out to be true. After getting a sketch from a witness, Espo and Ryan identify him as Vikram Singh, a "low-level analyst" for the Attorney General's Investigative Office in Washington, DC.
In retrospect, we find that Vikram is the person who calls Beckett at the beginning of XY, claiming a life-or-death situation using an AG team code. He says that he came across a declassified memo that was somehow connected to a search Beckett did two years ago while on the AG team; once the memo was sent on to her former team, that team died under circumstances which Vikram thinks were not accidental. Vikram also claims that someone tried to kill him the day before. He and Beckett go on the run to escape a team of operatives who appear to be after eliminating him and possibly Beckett as well.
Vikram has only worked for the AG for a few months as a "data cruncher", and apparently he's good enough to hack through the AG's "triple firewall" system to get a copy of the LOKSAT memo. When Assistant AG Hyde shows up in New York, she claims that Vikram forged his credentials in order to get his job and implies that he's the mole in the AG's office. Hyde is later killed and there's no reference to her allegations afterward.
Vikram's sister died when he was a kid, as he tells Beckett before they go to question Bracken.
In PhDead, we see that Vikram is now working for the NYPD, or at least for Captain Beckett, and seems to be helping her ferret out who's behind the LOKSAT memo and the deaths of her team. His name is mentioned in connection with various NYPD cases in the following episodes, and in The Last Seduction, Vikram advises Beckett not to have dinner with Castle (even though it's her anniversary).
When Beckett and Castle finally have a belated anniversary dinner and celebratory sex, Castle sees a text message on Beckett's phone while she's out of the room. It's from Vikram: "Urgent - I found something. Get someplace private and call me."
In the following episode, Mr. & Mrs. Castle, Vikram admits to Beckett that what he'd found was just another false alarm, and she accuses him of texting her to pull her out of her evening with her husband. Vikram continues to insist that Castle be kept in the dark about the LOKSAT investigation.
Vikram's talent for tech is shown in his skirmish with a hacker trying to find the location of his and Beckett's lair and data. The hacker, they find out, is Hayley, who launches a diversionary tactic in the form of cute cat videos. Vikram sends a command over the network that "nukes" Castle's system (fortunately, Beckett says that Castle has a backup).