Final answer:
The Secretary of State is the officer who holds the title of keeper of the Great Seal of Illinois.
Step-by-step explanation:
The officer who is the keeper of the Great Seal of Illinois is the Secretary of State. This role is significant as the Great Seal is used to certify official documents and legislation, ensuring their authenticity and formalizing their legal significance. In the context of the Illinois state government, the Secretary of State holds this important responsibility.