The truth was revealed after Makak was ranked number one in the local advertising magazine Campaign Brief Asia's annual rankings.
Rebelo said the pseudonym was "light-hearted" and was used as a way to credit the entire creative department's efforts, rather than just Greenaway himself, adding: "We are unapologetic about the incident."
But the row has sparked accusations that many of its recent campaigns were created purely for awards purposes. One commenter on Brand Republic wrote: "It's bad enough Saatchis Singapore wins awards for things such as lost dogs and martial arts schools, but going so far as to invent a person just so they can rise up the rankings ladder is lame."
Rebelo denied any allegations of scam advertising.
Greenaway was unavailable for comment.