Shopper is an agent, not a comparative. Many of the words ending in -er are comparative because they are based on a comparison like big, bigger, biggest, fast, faster, fastest. But when the word changes from an action to a person or noun then it is an agent like shopper, editor, creator, etc.
Visitor
he is a secret agent.
Dayf (visitor) Dyoof (visitors)
The visitor was kindly welcomed.
"Litter" is a noun and a verb and, as such, does not have a comparative degree.
Yes, the word visitor has three syllables. Vis-i-tor.
The rhyming word for superior visitor is competitor.
A comparative for the word ineffectual is more ineffectual.
smoother is comparative of word smooth.
The comparative is hotter.
comparative