In quoted companies is supposed to be the holder of Board integrity and the leader of the Board entity group. Should be independent, and accountable to shareholders, and ensuring the Board act legally with appropriate formalities. This role can be hijacked by powerful CEO's who insist on having total control. Recent attitudes to this aspect of over-concentration of power vary widely.
Either - a type of elder statesman who holds responsibility for the legal formalities and integrity relating to the operation of the Board. High level, balanced, experienced and calm. OR A raving megalomaniac who also wants to be CEO, COO, so no-one can affect their total control over the business. Your choice.