The Heart of Hearts, or Trueheart, is a physical manifestation of a person's soul. The very concept of a Heart of Hearts was introduced in The Gate of Genesis, where the final battle with Eclipse took place within his heart of hearts. It resembles a glass-like chamber that can be of any shape or size depending on the person it represents. Truehearts often are adorned with imagery that is said to reveal the true nature of the person. The conscious of a Trueheart lies in it's center, and can take the form of almost anything imaginable.
The Heart of Hearts and it's conscious can change forms, say if their holder contains other souls within his body, or is possessed or possessing another. Eclipse, being a titan of hatred in the body of a noble hero, has a Heart of Hearts filled with darkness, that covers up another Heart of Hearts filled with light. The soul of a person can freely move about in this chamber, and souls that a person absorbs can move around within as well.
Truehearts change and evolve over time as the people they represent grow and change.