Greenburg
Greenburg was a teacher on board the USS Enterprise-D in 2383, in an illusion created by Barash for William T. Riker.
Jean-Luc Riker was one of his students. He asked the boy about the Fornax Disaster, but Jean-Luc didn't know the exact stardate. (TNG: "Future Imperfect")
This character was only mentioned in dialogue.