Barbara already mentioned Michael Roarke from "The Disciple", but I'll mention him again. Thumbsup
"The Disciple" is clearly a "backdoor pilot" for a proposed TV series that never got off the ground. Perhaps more appearances would have helped?
Besides, Michael is part of the "exclusive club" that not only knows David's secret, but has even witnessed the "strange transformation" firsthand.

"Logical! If they were logical, they wouldn't be human!
They are driven by strange impulses"
General Urko (Mark Lenard), Planet of the Apes TV series, "The Surgeon". (1974).