The Waif was an acolyte of the Faceless Men serving in the House of Black and White in Braavos.
When playing The Game of Faces with Arya she claimed that prior to joining the Faceless Men she was the daughter of a widowed lord from Westeros who remarried another woman and produced a second daughter. Her jealous stepmother then tried to poison her. The woman who became the Waif discovered this plot and sought the help of the Faceless Men. She subsequently found her way to the House of Black and White in Braavos. Since the point of the Game of Faces is to mix lies with the truth convincingly we can assume that parts of her story were false and other parts were true.
The Waif has a sour demeanor and low opinion of Arya Stark when she arrives at the House of Black and White. When Arya presumes to claim that she is "no one" - a skill that the Faceless Men only achieve through dedicated training - the Waif becomes angered and beats Arya with her cane. She overpowers Arya and beats her to the floor before she can react. Jaqen H'ghar then enters the room and makes her stop. Jaqen is also critical of Arya, but more gently explains that she cannot truly be "no one" yet because she still thinks of herself as "Arya Stark", wearing Arya's clothes and carrying Arya's remaining possessions. Later, Jaqen leads Arya into the basement of the building where the bodies of those who come to seek the mercy of a quick death are taken. The Waif is there, and knows her way around well enough to undress and wash the body. Arya begins to help her, and asks what they do with the bodies after they are cleaned, but the Waif gives no response.
Later, the Waif obstructs Arya when she tries to walk through the door where the bodies are taken. When Arya queries about the fate of the bodies she has cleaned, the Waif tells her to be patient. When Arya tries to play the game of faces, the Waif harshly rebuffs her and orders Arya to return to work. Before the Waif can leave, Arya asks her who she used to be. The Waif claims that she was the daughter of a widowed Lord who remarried and produced another daughter. The jealous stepmother then tried to poison her but the Waif discovered this plot and sought the help of the Faceless Men. After telling her story, the Waif startles Arya by asking whether she believed her. When Arya expressed surprise, the Waif responds that Arya must be able to lie convincingly in order to pass the game of faces.
When Jaqen sends Arya out on her first assignment to deliver a gift of the Many-Faced God to the Thin man, the waif expresses doubt as to Arya's readiness and chance of success. Instead of assassinating the Thin man, Arya murders the Kingsguard Ser Meryn Trant to avenge the death of Syrio Forel. The Waif and Jaqen intercept Arya when she tries to return a mask to the Hall of Faces. By killing the wrong man, Arya has cheated the Many-Faced God. The Waif gloats over Arya's failure and grabs her while Jaqen drinks a vial of poison and collapses.
The Waif watches in elation while a distraught Arya weeps beside the body of Jaqen. When the Waif asks why Arya is mourning, she replies that Jaqen was her friend. Suddenly, Arya hears Jaqen's voice saying "he was no one." Arya then turned around only to find Jaqen standing in place of the Waif. As punishment for taking the wrong life, Arya is rendered blind.
After Arya is blinded and reduced to begging on the streets of Braavos, the Waif forces Arya to fight her with two fighting sticks. Due to Arya's blinded state, the Waif gains the upper hand over Arya and leaves her beaten and bruised. Before leaving, she tells Arya that she will be back the following day. The following day, the Waif revisits Arya and asks her name. Though Arya replies that she is "no one", the Waif is still unconvinced and beats her a second time. In response to the Waif's taunt, Arya tries to fight back only to find that the Waif has been replaced by Jaqen H'ghar. After testing Arya three times, he deems her worthy to rejoin the Faceless Men and takes her back to the House of Black and White
Later, the Waif continues training Arya in the House of Black and White. The Waif plays the game of faces with Arya, questioning her about her family, the Hound and the people on her kill list. Arya recites her list for the Waif who comments that it is short and cannot be everyone she wants to kill, she then queries if Arya is forgetting 'someone', implying herself, Arya then asks which name the Waif would like her to speak suggesting the Waif is not on her list because Arya does not know her name. The Waif spars with Arya on a large stone tablet, during which the Waif hits Arya with a fighting stick despite Arya having no vision, these fighting lessons are suggested to be going on for at least several days. At first the Waif constantly outmaneuvers her repeatedly knocking her to the ground however, eventually Arya manages to respond and defends herself with her own fighting stick to the point where the Waif becomes visibly distressed. When Arya lands a blow of her own the Waif seemingly becomes overwhelmed with anger and makes a rash move which Arya intercepts whilst Jaqen observes. The Waif then reluctantly leaves Arya alone with Jaqen, who restores her sight after deeming her worthy of becoming a Faceless Man.
The Waif fights against Arya with staffs in the temple. She defeats her foe easily; at a certain point, she lays down her weapon. However, the Waif still manages to outmaneuver Arya and punches her to the ground announcing that "Lady Stark" will never be one of them. Jaqen enters soon afterwards, and the Waif takes her leave. The Waif is then sent by Jaqen to keep an eye on Arya as she is tasked with assassinating Lady Crane. After Arya fails the Waif has a conversation with Jaqen, suggesting that they came to a deal allowing the Waif to kill Arya for her failure. After hearing Jaqen's request to kill Arya humanely, the waif sets out to fulfill her mission.
The Waif eventually manages to locate Arya, and approaches her wearing the face of a feeble old woman. Before Arya can react, the Waif pulls out a dagger and repeatedly stabs Arya in the stomach, critically wounding her. Arya manages to momentarily escape by jumping into the canal, though her wounds begin to bleed out
The Waif kills Lady Crane, who had been sheltering Arya, and pursues an injured Arya throughout Braavos. Arya leads the Waif to her hideout, where she finds Needle and cuts the candle that provides the only light in the room. Arya, having experience in fighting without sight, is victorious, and adds the Waif's face to the Hall of Faces.
The Waif was an acolyte at the House of Black and White who never believed Arya Stark was a worthy addition to the faceless Men. After witnessing Arya sabotage her own assignment, the Waif hunted down the Stark girl, but lost the faceoff.