T'Hain was a Vulcan writer, the author of Dictates of Poetics.
A quote from the book stated: "A character's actions must flow inexorably from his or her established traits."
Tuvok quoted T'Hain in 2373 when Tom Paris wanted to make revisions to Tuvok's Insurrection Alpha holoprogram that were at odds with the characters' personalities, such as Kathryn Janeway executing a prisoner. (VOY: "Worst Case Scenario")
This character was only mentioned in dialogue.