From A Wiki of Ice and Fire
A regent is in charge of ruling the realm when the current king is underage. The regent's rule ends on the the king's sixteenth name day.
- Eddard Stark in the will of Robert Baratheon was named 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.