Dr. Diamond was an anthropologist that Agents Dana Scully and Fox Mulder spoke with on their investigation into the Jersey Devil. He went on to directly help them in their attempt to capture the creature due to the beast's relevance as a scientific discovery.
Advising the Agents
Scully brought Mulder to Dr. Diamond with the clear intention of proving to him that the Jersey Devil was a pure fantasy. Diamond explained that every culture had a myth like the Jersey Devil, like Yetis or Russian Almas, but that it was based on Human fear about their dual nature as creators and destroyers of life. Dr. Diamond talked with Mulder about the nature of Humans as a species and their destructiveness as they spread over the world. Though Scully had wanted Dr. Diamond to disagree with Mulder's hypothesis, Diamond ended up agreeing with Mulder due to Mulder's knowledge on the subject of Humans reverted to animal instincts.
Aiding the Agents
Later, Dr. Diamond accompanied Mulder, Scully, and Ranger Peter Brullet in their attempt to capture the Jersey Devil. When Detective Thomas Tomson's SWAT team caught up with Mulder's team during their hunt of the creature, Diamond was unwilling to give away Mulder's location or admit to knowing Mulder to protect Agent Mulder. Diamond was there with Mulder when the Ranger got her with the tranquilizer gun but they failed to capture her alive. The dart didn't take her down quickly enough and she escaped Mulder's team. Tomson's team had killed her when she attacked one of his men.
Dr. Diamond was played by Gregory Sierra. Here is his imdb page.