Serena

The Lady Serenadellatrovella (or Serena for short) was a Time Lady assigned by the Celestial Intervention Agency to work with the Doctor in preventing the Players from changing history. Serena sacrificed herself to save the Duke of Wellington, who the Player called Valmont was attempting to shoot. The musket ball tore through both of her hearts, killing her instantly and making regeneration impossible. (PDA: World Game)

Behind the scenes
Serena worked with the Doctor in the Season 6B continuity.