Properties on Zillow can have a section called What I Love About the Home where either the homeowner or the list agent can manually enter additional information about the property that they feel may be of interest to potential buyers. The text in this section does not appear anywhere on the listing on the MLS- it is a completely separate, extra field added and edited through Zillow.
If the homeowner claims the property page on Zillow, they can access the What I Love section.
For the list agent to do it, they would need to create a Zillow profile with the same email address that is used on the listing feeds. The listing will then be claimed to their profile and they can edit the listing and add to the What I Love section.
If the agent does not want to create a Zillow profile, Zillow support can update the section manually if they are sent the comments to be added to What I Love.