Helen Walker (Sophie Colquohoun) is the main villainess of "Ye of Little Faith", episode 3.04 of BBC's Death in Paradise (airdate February 4, 2014). She worked as a flight attendant in Saint Marie's airport and was the girlfriend of pilot Adam Frost.
Prior to the episode, however, Helen had been overhearing rumors (later proven truthful) along the cabin crew that fellow flight attendant, Natasha Thiebert, was having a fling with Adam. This was when Helen decided to get rid of her love rival by killing her.
Helen decided to kill Natasha by poisoning her in a clever way—by lacing a strip of postal stamps with bikh, a deadly posion, and giving it to Natasha, knowing she would use them to mail a postcard home. By the time Natasha licked a stamp to place on the envelope and succumbed to the poison, Helen was already on a flight, outwardly giving herself an apparently solid alibi.
However, when DCI Humphrey Goodman and DS Camille Bordey confronted Adam regarding his relationship with Natasha, Helen reacted in a displeased manner upon finding out that the affair was true, causing the police to suspect that Helen was the killer. Helen's undoing was when she planted the vial of poison in the crime scene to frame Adam. When the police decided to reinvestigate the scene, Goodman found the vial. The vial of poison contained a hair, as goat hair that was used for the makeup brush Helen used to put poison on the stamp were left behind on the stamp. The hair would eventually contain her DNA, incriminating Helen. At the denoument, Helen was confronted with these evidence, with Goodman revealing how Helen had attempted to frame Adam for Natasha's murder after learning that he had in fact had an affair with Natasha. Following the summation, Helen was placed under arrest for killing Natasha.