From A Wiki of Ice and Fire
A regent is in charge of ruling the realm when the current king is underage or incapable of ruling due to injury or illness. The regent's rule ends on the the king's sixteenth name day or when the king recovers from his injury. All known regents have also taken the title Protector of the Realm, a title usually reserved for the king.
- Prince Aemond became Prince Regent after his brother Aegon II was severely wounded and incapacitated for a year after the Battle of Rook's Rest during civil war known as the Dance of the Dragons.
- Eddard Stark in the will of Robert Baratheon was named Lord Regent to Joffrey Baratheon but his regency never transpired.
- Cersei Lannister declared herself Queen Regent to Joffrey after Eddard Stark was arrested, and after Joffrey's murder named herself regent to Tommen Baratheon.
- Kevan Lannister is made Lord Regent to Tommen after Cersei's imprisonment by the Faith of the Seven.