Surely this red one was the devil, but my devil. That is, he was my joy, the joy of the serious person, who keeps watch alone on the high tower — his red-colored, red-scented, warm bright red joy.
Jung identifies joy as the shadow-companion of the serious, individuating self — a devil-figure that arrives unbidden, causes self-forgetting, and is inseparable from depth and seriousness.
, The Red Book: Liber Novus, 2009thesis