Bridget Regan as Merlyn.

Eoin Macken as Gwaine.

Blathnaid McKeown as Hope Lotsson.

Baylen Bielitz as Balinor Lotsson (as he gets older).

Lady Caroline Faber as Hunith.

Richard Wilson as Gaius.

Bradley James as King Arthur Pendragon.

Angel Coulby as Guinevere.

The Knights of the Round Table are:

Rupert Young as Sir Leon.
Tom Hopper as Sir Percival.
Santiago Cabrera as Sir Lancelot,
Harry Melling as Sir Gilli
Warrick Davies as Sir Grettir
and Tomiwa Edun as Sir Elyan

Emily Blunt as Esmerelda. Gwaine's older sister married to Lord Bayard's son, Benjamin.

Michelle Pfeiffer as Lady Adelaide Lotsson. Gwaine and Esmerelda's mother.

Domhnall Gleeson as Prince Benjamin 'Ben' Collings of Mercia

Clive Russell as Lord Bayard Collings of Mercia