Christina Lang (Helen Johns) was a hidden villainess from "Welcome Back Crocker", episode 5.13 of Republic of Doyle (airdate January 22, 2014). She was a Crown Attorney who first met with Sergeant Leslie Bennett asking about the whereabouts of Inspector Craig Smallwood, him having missed a meeting they had.
As already revealed, Craig had been kidnapped by escaped felon Kevin Crocker, who forced him to work with Jake and Malachy Doyle to prove his innocence in the murder of prostitute Lisa Younghusband. As revealed later on in the episode, Lisa was alive, and had been paid by Christina to disappear from town as part of a conspiracy to frame Kevin for her "murder". Christina also revealed to be conspiring with Judge Richardson (during his days as an attorney) to throw out drug-related convictions, likely working alongside drug dealers in exchange for profits.
When Christina learned that Craig Smallwood was investigating the conspiracy and was close to learning the truth, the villainess paid her henchman, Brian Mitchell, to kill Smallwood, with Mitchell planting a bomb on his car that killed him when it went off. Christina was revealed as a villainous conspirator when Des Courtney found a security camera photo of Christina buying a burner phone for Mitchell. After learning Christina's role in the drug conspiracy and the conspiracy to frame Kevin, Jake went with Kevin to confront Christina, only to find her strangled to death in her home. Her killer was ultimately revealed to be Mitchell, who he later described as a "liability", while also revealing that she was supposed to kill Lisa Younghusband rather than pay her to leave town.