In the army lists, later in this book, you'll find characters you can field as part of your army. As with squads of troops you can choose for your army, the character's profile tells you how tough he is and what weapons and armour he can have. It is left to you to name your heroes, and follow their illustrious careers across bloody battlefields.
Nonetheless the 41st millennium is filled with famous characters renowned as legendary heroes or infamous villains. These individuals are called special characters and can be found in the Codex army books and in White Dwarf magazine.
Special characters are highly skilled and dangerous opponents who have special traits or skills that make them particularly valuable to an army.
For example, Lord Commander Solar Macharius is without doubt the finest Imperial Guard commander who ever lived, his armies conquered nearly a thousand worlds in under seven years. An army led by him can expect great benefits in strategy and tactics if little direct help in fighting. On the other hand Khârn the Betrayer, an exalted champion of the Dark Gods, is a mad butcher and slaughterer of men with no thought for anything other than personal combat (which it must be granted, he excels at).
Special characters represent unique individuals, past and present, that many players like to use in their games to give background or history to their army. However, these powerful characters can dominate certain types of game, especially the smaller battles. It is general practice that special characters can only be used if both players agree to do so before the battle.