A mystery man who appeared in a Facebook photo kissing a missing teenager says he was "blown away" when his cousin sent him a link to a recent article about the disappearance and asked him "what's going on bro, this is you".

Katrina Bohnenkamp, 15, was last seen in the early hours of October 26, 2012, at a boarding house where her father lived in the Sydney suburb of Strathfield. Three months earlier, she had uploaded a photo to her Facebook profile which showed her kissing an unidentified man.

Last year, at the opening of an inquest into Katrina's disappearance and suspected death, investigators urged anyone with information about the man to come forward. No one has ever been charged over Katrina's disappearance and there is a $500,000 reward.

On Monday, Darren Denning wiped his eyes as he told the NSW Coroners Court that he was the man in the photo and he had no idea Katrina was missing until his cousin contacted him several months ago.