Unella is a Septa of the Faith of the Seven and a devoted follower of the High Sparrow.
Unella grabs Cersei Lannister when she is arrested by the High Sparrow and throws her into a cell underneath the Great Sept of Baelor with the aid of Moelle and Scolera.
Unella routinely visits Cersei in her cell and offers her a ladle of water on the condition that she confess. Cersei, prideful and infuriated, refuses Unella's advances, reminding the septa that her face will be the last one she sees before she dies.
Eventually, however, Cersei gives in and confesses her adultery with her cousin Lancel. The High Sparrow allows Cersei to return to the Red Keep in order to be with her son King Tommen, but first she must perform a walk of atonement. Unella is present when Cersei is stripped naked and washed by two other septas before her hair is roughly cut short. She later accompanies Cersei as she is forced to walk naked from the Great Sept of Baelor to the Red Keep with a handful of Faith Militant members to keep away the leering and jeering crowds who have gathered to see her shame and take the chance to insult a Queen. As she walks, Septa Unella accompanies her crying out "Shame!" repeatedly while ringing a bell to attract the attention of the population.
While Septa Unella reads from the Seven Pointed Star regarding sin to an imprisoned Margaery Tyrell, the latter inquires about her brother Loras Tyrell's condition, who was also imprisoned too. However, Unella says she must confess her sins first. When the Queen demands to know the condition her brother is in, the septa moves to strike her, much to Margaery's horror. At that precise moment, the High Sparrow steps in and stops Unella, ordering his acolyte to attend the Faith's 'other guests'. After the septa leaves, he admits that at times Septa Unella takes her devotion too far.
Some time later, Unella brings Margaery to the High Sparrow. When Margaery correctly recognizes the High Sparrow paraphrasing his life from the Book of the Stranger, he expresses surprise, and Margaery explains that Unella frequently reads the Book at her. The High Sparrow chuckles and agrees that Unella has a habit of reading at people instead of to them.
Unella is present on the steps of the Great Sept as Margaery is about to do her walk of atonement, likely with the intent of again ringing the bell. She remains silent throughout the Tyrell army's aborted rescue attempt and Tommen's announcement of a new alliance between the Faith and the Crown.
Unella has appearently been assigned to watch over Queen Margaery non-stop. The septa is naturally present when Olenna and her granddaughter are finally reunited, much to the Tyrell matriarch's ire. The Queen Of Thorns wastes no time in telling Unella off and even tries to intimidate her. Unella is made of sterner stuff though and remains implacable. When Margaery reveals she is 'truly' devoted to The Faith, to her grandmother's shock a hint of a smile appears across Unella's lips.
After the destruction of the Sept of Baelor by Wildfire, Cersei captures Unella and imprisons her within the Black Cells. The queen confesses that she killed Robert Baratheon, that she slept with her brother, and that she destroyed the Sept, killing the High Sparrow and the Faith Militant. She then introduces Unella to Ser Gregor Clegane, who proceeds to slowly torture Unella to Cersei's delight.
The brutish jailer who oversaw Cersei's confinement, Septa Unella finally extracted a confession from her prisoner. Cersei got her revenge by imprisoning Unella after the explosion at the Sept of Baelor - and putting Ser Gregor Clegane in charge of her captivity.