Andrus Hagan

From Memory Alpha, the free Star Trek reference.

Andrus Hagan was a Betazoid science adviser who worked for Starfleet in an apparent civilian capacity aboard the starship USS Brattain in 2367.

When the ship fell prey to a Tyken's Rift and caused her Human crew to go insane and kill each other, Hagan was the sole survivor. An away team from the USS Enterprise-D found Hagan, himself either insane or rendered mute. Deanna Troi feared the same fate would befall her if the phenomenon happened aboard the Enterprise. (TNG: "Night Terrors")

Andrus Hagan was played by John Vickery.