Eustace

Septon Eustace was a member of the Faith of the Seven serving in the Red Keep during the Dance of the Dragons.

History
Septon Eustace crowned Prince Aegon Targaryen, son of Viserys I Targaryen and Alicent Hightower, as Aegon II Targaryen.

In the books
In the A Song of Ice and Fire novels, there's no mention of Septon Eustace. He does appear in The Princess and the Queen and The Rogue Prince novellas. Eustace wrote The Reign of King Viserys, first of His Name, and the Dance of the Dragons That Came After, one of the sources for later historians who wrote of the war.

He crowned Aegon because the current High Septon was too frail to travel from Oldtown - then the seat of the Faith - to King's Landing

However, Eustace's accounts tended to be biased in favor of Aegon II, putting him in a positive light, and against Rhaenyra Targaryen. It is believed he invented several details, such as Aegon being relucant to take the crown until Alicent reminded him that Rhaenyra would execute him and his family if she sat the Iron Throne - as other accounts note that Aegon was "grasping" and compare him to other "bad" kings such as the weak Aenys and the tyrannical Maegor the Cruel. Eustace's accounts also note that Rhaenyra cut her thighs when sitting the Iron Throne - a sign that she was unworthy of it - even thoughit was physically impossible since she was wearing full armor at the time.