Legal Question in Intellectual Property in Illinois

Re: Existing Characters used outside of the advertiser

Your question involves issues in trademark and copyright law. The owner of the trademark in the character(s) is probably be the advertiser.

I'm not sure what the phrase "originally created by an ad agency" means, unless you are suggesting that the copyright may be held by the agency, and not the advertiser. (A review of the ad agency-client agreement would probably be required.)

If your question involves characters from an ad for a product directed at children, there are rules in children's advertising that restrict placement of such ads with shows featuring the same characters--it's too confusing for children