Peace is the personification of peace, akin to the personifications of Death, War, Famine and Pestilence who make up the four riders of the apocalypse, and four of the five founding members of The Horsemen.
She is mentioned in the 2011 Hallowe'en event, in a carving found when examining a tree on Death's Citadel. The carving "War + Peace XX" in a heart implies that War and Peace used to be involved romantically.
In the 2018 Hallowe'en event, War confirms that he and Peace used to be romantic partners but split due to their opposite inherent natures.
She does not appear in game.