The High Sparrow is the leader of the religious sect known as the Sparrows and the newly appointed High Septon.
His name is unknown. He was originally a humble septon who began to preach equality among all men and aid the poor while leading a humble life himself, often denouncing the excesses and sins of the nobility. As followers gathered around him, he made his way to King's Landing.
He emerged as more or less the leader of the religious protest movement known as "the Sparrows", and his critics started mockingly calling him the "High Sparrow" - in sarcastic comparison to the High Septon, the highest cleric and ordained leader of the Faith of the Seven. His followers, however, adopted the nickname in pride, and themselves started calling him "the High Sparrow" as a sign of respect. He never tried to call himself this, but the name stuck. (After being made High Septon, as per tradition it became forbidden to ever refer to him by his given name again, so whatever his original name was remains unknown).
Although the High Sparrow seems a harmless old man, he has a steel personality. He does not bow down to authority, and cannot be threatened or intimidated.
Queen Cersei Lannister discovers the High Sparrow is a humble man, serving soup and bread to the poor while barefoot. He finds the name "High Sparrow" humorous and makes no effort to hide what action the Sparrows took against the High Septon, though he admits they could have been more careful. Cersei tells him that he will not be arrested or executed, but the High Septon has been imprisoned instead, which surprises the High Sparrow.
Through Queen Cersei's influence, the High Sparrow is elected High Septon. To further gain his support, Cersei signs a decree restoring the Faith Militant, military orders under the command of the High Septon with the right to bear arms and dispense justice.
The High Sparrow later presides over the inquest into the charges of sodomy, buggery, and blasphemy against Loras Tyrell. He also questions Margaery Tyrell and Olyvar on the charges. Olyvar's testimony convinces the High Septon to bring Loras to a full trial, and also to arrest Margaery for perjury and blasphemy, as she swore before the gods that her (false) testimony was true.
Whilst scrubbing the floors of the Great Sept, the High Sparrow is approached by Olenna Tyrell. After exchanging some barbs and pleasantries the pair discuss Loras and Margaery's arrests. The Queen Of Thorns wants her grandchildren released immediately but the High Sparrow refuses arguing that the divine laws should apply to both high-and lowborn. Olenna resorts to offer bribery and when this fails she threatens to have House Tyrell cease supplying the realm with their crops. Unimpressed, the High Sparrow sarcastically questions the Tyrell matriarch if she ever performed a labor in the field. He adds that the smallfolk are many while the nobility are few and what will happen when the many stop fearing the few. He then turns around and leaves her.
Late, the High Sparrow receives Cersei, who has come to the Great Sept to see the imprisonment arrangements of Queen Margaery, at the lowest and oldest level of the Great Sept - a simple stone sept with none of the grand adormnents of the Great Sept that Baelor the Blessed built on top of it. He tells her of the limited known history of the chapel, making plain his love of the simplicity in how it represents pure faith and compares it to the gratuity of the Great Sept. He uses this comparison to show how the highborn, like the Tyrells, can be stripped back to reveal the pure truth beneath their facade and then openly wonders and asks what they will find of Cersei once they strip back her finery to reveal the truth beneath. He then tells her of a highborn young man who has already stripped himself this way and add that he had much to say of her. Lancel Lannister then enters the chapel to stand beside the High Sparrow. They both silently watch as Cersei's is arrested by Septa Unella and brother Boake.
His High Holiness agrees to receive Cersei when she finally consents to confessing her sins. He interrupts Cersei's long speech of wanting to be pious again in order to hear her actual confession. The High Sparrow is relatively pleased that the Queen admits to having committed adultery with Lancel, but states that a trial will still be held to ascertain the truth behind the other charges she still denies: regicide and incest. When Cersei invokes the Mother's mercy, the High Sparrow agrees to let her return to the Red Keep to see Tommen but only after she has atoned. Before the gates of the Great Sept the High Sparrow addresses the people of King's Landing. He explains that though their Queen has sinned greatly she now begs forgiveness and will thus perform a walk of atonement from the Sept to the Red Keep as the gods made her, meaning completely nude and with her long golden hair cut short. When the once prideful Queen walks past him, the High Sparrow grins ever so slightly.
He stops Septa Unella from physically chastising Queen Margaery. Though he refuses to answer Margaery's questions regarding her brother's wellbeing, he does reveal King Tommen misses her. The High Sparrow believes the love between man and wife is sacred, but sin can cloud this. When the Queen refuses to confess to any wrongdoing, he ask whether Margaery truly considers herself to be without sin of any kind. He is pleased when the the Queen at least allows that everyone makes mistakes.
The Sparrow walks in on Jaime and Tommen making amends. The King demands to speak to his queen. He informs Tommen that as long as Margaery hasn't confessed and atoned she will remain in seclusion. After Tommen takes his leave Jaime and the High Sparrow first discuss funeral customs. Jaime is livid that the Faith took it upon itself to humiliate his sister, but he quips that Cersei sinned greatly and paid the price. Boldly Jaime demands to know whether his own sins can be forgiven and moves to kill the leader of the Faith. Unfazed the High Sparrow declares that though each member of the Faith is small and insignificant, united they toppled an empire. Jaime soon realizes he's surrounded by the Faith Militant and is forced to stay his hand.
Later, the High Sparrow has an audience with King Tommen, who petitions for his mother Cersei to be granted permission to visit her daughter Myrcella. The Sparrow declines the King's request on the grounds that Cersei has not been cleared of her other crimes; namely the murder of King Robert Baratheon and incest. When Tommen criticizes the Sparrow's legalistic treatment of his mother, he professes admiration for the deep love between mother and son, which he attributes to the Mother. However, he assures Tommen that he has no vendetta against his mother but is merely carrying out the will of the Seven. Before Tommen leaves, he tells the King to pray to the Seven and that the gods have worked through his grandfather and mother even though they may not acknowledge it.
The Sparrow summons Queen Margaery. When the queen enters, he asks her where she would go if he were to release her. When Margaery truthfully answers she would return to her family, the Sparrow warns her that this would lead her to seek out sin again. The High Sparrow tells Margaery his story on how and why he became so pious and devoted to the Faith of the Seven. The Sparrow then allows Margaery to visit her brother.
Tommen comes inquiring about Margaery's wellbeing yet again. The High Sparrow is quite accommodating and admits The Queen has always stood by common people and is now showing signs of having embraced the Faith. His High Holiness finally allows Tommen some time alone with his wife. When the Tyrell army lead by Jaime and Mace come to demand Margaery and Loras' release the Sparrow initially refuses even in the face of death. After some tense moments his High Holiness surprises all present by stating that Margaery won't be performing a walk of atonement. The Queen has shown true devotion to the Seven and has even brought Tommen into their ranks, so there is no need for punishment. As Lord Mace and Jaime skulk off The High Sparrow is beaming triumphantly.
His Holiness walks in on Queen Margaery piously reading from the Seven Pointed Star. After briefly discussing the The Mother, the High Sparrow asks Margaery if they can discuss a personal matter. Tommen has told the Sparrow that since his wife's release, they have yet to share the marriage bed. Margaery tries to explain that she is just treading carefully to avoid relapsing into sin. The Sparrow reassures her that a wife giving her husband an heir is no sin; it's her duty. Next, the pair discuss Lady Olenna. Though his High Holiness feigns admiration for the Tyrell matriarch, he threatens her in the very same breath. After all, the 'Queen of Thorns' is an unrepentant sinner.
It appears as though his High Holiness now wields significant influence over King Tommen, and is fast becoming the de facto ruler of King's Landing. After Cersei refuses to speak with him, ordering Gregor Clegane to kill a fanatic in the process, The High Sparrow 'convinces' Tommen to abolish trial by combat via royal decree as it is considered 'cheating'. Henceforth, all trials will be presided over by seven members of the Faith, as it was in the days before House Targaryen conquered the Seven Kingdoms.
On the day of Cersei's and Loras' trial, the High Sparrow and many of the city's elite gather at the Great Sept of Baelor. Loras is brought forward and confesses to his homosexuality and begs to atone by giving up his name and titles as the heir of House Tyrell. However, Cersei fails to appear to the trial, and the High Sparrow sends Lancel to retrieve her. Margaery becomes suspicious of both Cersei and Tommen's absence and tries to warn the crowd to leave, but the High Sparrow dismisses these suspicions and prevents the audience from leaving, having his sparrows block the doors. The wildfire cache is then ignited and completely destroys the Great Sept, killing countless people including the High Sparrow, Margaery Tyrell, Loras Tyrell, Mace Tyrell, Kevan Lannister, and Lancel Lannister.
A wealthy but lowborn cobbler, the High Sparrow swore off earthly possessions and devoted himself to the Faith of the Seven, the dominant religion of Westeros. His unassuming appearance contrsted with his powerful position as head the Faith. With the permission of Cersei Lannister, the High Sparrow reinstated the Faith Militant, a once-banned religious order that fanatically guarded the purity of the Faith and administered the justice of the gods. He then engineered an alliance with King Tommen and Queen Margaery, increasing his power. Present in the Sept of Baelor for Cersei's trial, the High Sparrow was among those killed in the explosion.
The High Sparrow presents himself as humble, pious and wise. He is unmoved by life's luxuries and abjures materialism. While some other septons are indifferent toward the small-folk and their problems, the High Sparrow consistently shows concern and compassion.
However, behind a harmless facade, he is ruthless and fanatical in his beliefs as well as a shrewd and highly intelligent political player and Machiavellian with a keen and personal understanding of the narcissistic mindsets of many of the political players of Kings Landing. He manipulates Cersei's resentment toward Margaery to gain control of King's Landing through the Faith Militant. Later, he gains influence over the naive and weak-willed King Tommen Baratheon, obtaining even more power through deliberate manipulation and exploitation of the young king. He also psychologically tortured Cersei, Loras, and Margaery to obtain a confession on their "sins".
It's unclear if he really believes in his piety or if his humility is only a facade to obtain power. He has shown to display a smug smile when he knows he has the upper hand in a conflict, and he displays a rather arrogant certainty in his interpretation of the will of the Gods. Even if this is so, whether he does it for a genuine greater good strengthened by faith or for his own political agenda and hubris - perhaps a combination thereof, as is often the case with those of similar ambitions - is unclear. Despite this, he appears to be unafraid of death, remaining calm when Jaime threatens to cut him down, though this could just be because he gauged, quite correctly, that Jaime would not kill him if it were to mean his own death afterwards although he did seem shocked when he discovered that Cersei had laced the Great Sept of Baelor with wildfire just before his death.
However, despite his brilliant manipulations and his army of followers, the High Sparrow's blind devotion ultimately led to his death. As Cersei put it; because he had no use for anything in the material world, he was unafraid of any threat the material world could produce. Somewhat similar to Daario Naharis's taunt against Grey Worm, a lack of fear limits a person's understanding as to how scared people think, thus making it difficult for them to manipulate those people. The High Sparrow was over-confident that Cersei was powerless against himself and his followers. Therefore, he was unable to comprehend how extreme her actions would become in order to eliminate them (i.e. blowing up the Sept of Baelor with wildfire and killing dozens - if not hundreds - of innocent people in the process).