Ensign Davis was a Starfleet engineer and member of the USS Enterprise-D crew sometime between 2366 and 2368. Davis was present during an accident in main engineering and was one of the last personnel to evacuate before the isolation door sealed the warp core off from the rest of the ship. He thought Ensign Keller was right behind him, but she was apparently injured and left behind when Commander Riker gave the order to lower the door.
Riker re-experienced this event during a telepathic memory invasion caused by the Ullian Jev. In this hallucination, Davis blamed Riker for Ensign Keller's death, and later became Jev himself. (TNG: "Violations")