Jonek was the name of a woman that was one of the false memories planted into Geordi La Forge's mind after being brainwashed by the Romulans. He believed he had met her on Risa.
When trying to recall these memories in 2367, he started to tell Deanna Troi about Jonek, but felt she wouldn't want to hear the details. (TNG: "The Mind's Eye")
This character is only mentioned in dialogue.