"High Septon" is the title given to the head of the Faith of the Seven, the dominant religion of the Seven Kingdoms. It is a position of supreme authority within the Faith, but the holder has little power outside of the religion. This High Septon was appointed after his predecessor was killed in the riots of King's Landing.
This High Septon was appointed after his predecessor was killed in the riots of King's Landing. Pycelle reports consulting the High Septon about King Joffrey Baratheon setting aside his betrothal to Sansa Stark
The High Septon talks about an orphanage in Flea Bottom to Margaery Tyrell. Margaery then stops by the orphanage while travelling from a sept to the Red Keep to talk with the orphans and give them some toys, surprising her betrothed Joffrey Baratheon
The High Septon conducts the wedding of Tyrion Lannister and Sansa Stark at the Great Sept of Baelor.
The High Septon conducts the marriage of Joffrey Baratheon and Margaery Tyrell.
He is present in the Great Sept of Baelor after King Joffrey's funeral, when Cersei Lannister, Tommen Baratheon and Tywin Lannister are privately mourning the death of the King before his body. After Tywin takes Tommen away, Jaime Lannister arrives and asks the High Septon to give the Queen a moment alone with her dead son, prompting them all to leave. The High Septon quickly complies and leads the attendant servants out of the Sept.
During Tommen's coronation in the Throne Room, the High Septon places the crown upon his head, officially announcing him as the new King
The High Septon later stands outside the Sept of Baelor immediately preceding the funeral of Lord Tywin. He suggests to Cersei that they start soon, since lords and ladies have come from all over Westeros, but she says they will wait a little longer while she has a moment alone with Jaime and Tywin's corpse
The High Septon conducts the wedding of King Tommen to Margaery Tyrell. Some time later he visits Littlefinger's brothel, of which he's an recurrent patron, and engages in a sexual roleplay where the Father, a disguised Olyvar, rewards him by offering him one of six other members of the Seven to "worship". The High Septon chooses a prostitute portraying the Maiden, which he seems to constantly pick, as well as one portraying the Stranger. As the two whores approach him, Lancel Lannister along other Sparrows break into the chamber, attacking Olyvar and scaring the prostitutes away. The High Septon is aghast by the attack, but he's soon stripped of his clothes and paraded as a sinner through the streets of King's Landing; with a sparrow beating his hands when the High Septon tries to cover his genitalia. Beaten and humiliated, the High Septon approaches the Small Council and demands the execution of the Sparrows, particularly their leader, citing that an attack on his person is an attack on the gods themselves. Qyburn reveals to the council where the High Septon had been attacked, to which the old man replies he had been preaching to the whores for their salvation. Queen Cersei has him thrown into the dungeons of the Red Keep.
The High Septon headed the Faith of the Seven until the Sparrows, led by Lancel Lannister, exposed him as a patron of Littlefinger's brothel. After being paraded naked through the streets, the High Septon appealed to the Small Council for justice. Cersei had him jailed instead.