The High Septon is a high priest of the Faith of the Seven. He would originally reside in Oldtown until the invasion of Aegon I Targaryen, when he moved to King's Landing. They currently reside in the Great Sept of Baelor in King's Landing.
The High Septon wears a large crown made of crystal. He gives up his family name and any family connections, as he is now seen as an avatar of the Gods.