Michael Sophocles, who appeared on the 2008 series of the business show, described the Madonna look-alike as "a nymphomaniac" and revealed that she attempted to sell a 'kiss and tell' story about their relationship.
"She wasn't well-known but she made it her business to tell people how talented she was and how successful she was going to be," he told the The News of the World.
"She was a good-looking girl, nothing to write home about, but she dressed in a sexy way. It was obvious she was ruthlessly ambitious. She liked being out with me because of the attention I got.
"She told me there was a connection the first time we met. She kept asking me to take her to all these big parties and clubs. It was obvious what she wanted. She was infatuated with the idea of celebrity."
Sophocles added that Waissel was so desperate to show off her talent that she sung for him during sex and forced him to watch recordings of her vocal performances.
"Katie was lying on top of me and said, 'Guess what? I've written a song about you.' And she sung it to me there and then. It was all very arty and affectionate - but far too much.
"After we did the deed we played music and danced around in our underwear like mentalists. Then Katie made me sit in front of my TV and watch her singing. She'd brought a DVD of a concert her dad fixed for her. She was a fantastic singer but the situation was so cringe-worthy."
Sophocles confessed that their relationship took a turn for a worst after Waissel caught him cheating with another woman, but their romance eventually ended after she later got involved with Death at a Funeral dwarf actor Jordan Prentice.
"I went to the toilet and came out to find them snogging, tongues and everything," Sophocles said. "She was all over him. That was it for me."
Waissel, who reportedly dated singer James Blunt after her split from Sophocles, starred in an online reality show centred around her life before entering The X Factor.