Grimaud was a Betazoid Augment who lived on planet Tagra IV in the late-24th century. He was named after the character in The Three Musketeers who rarely ever spoke, but communicated through sign language. He chose to communicate telepathically with words and mental images rather than speaking. For example, he transmitted an image of flowers to Aurellan Markalis to indicate he was smitten with her.
Through genetic modifications, Grimaud had enhanced telepathic abilities which he could not always control. He used telepathic inhibitors and Vulcan meditation techniques to quiet the voices in his head. He had been broken out of a mental institution by Rhys Darcen, who believed the doctors there wanted to completely suppress Grimaud's telepathic abilities. Of course that claim turned out to be false when the USS Lambda Paz EMH Mark III learned that the doctors were helping Grimaud better control his abilities, especially not to read the thoughts of others without permission. While Darcen dreaded the idea of someone involuntarily probing his thoughts, he also believed Grimaud's gifts should not be wasted. (Star Trek: Lambda Paz: "Midnight Ride", "Religion To Do Good", "A Cause of Greater Worth")
Grimaud's telepathic abilities extended to being able inhibit all bodily motion. He exercised that ability to prevent Limis Vircona from escaping Darcen's custody, as well as Darcen and Ileana Roshanak from trying to kill each other. Markalis was then able to convince Grimaud to free them, citing his earlier negligence to prevent Limis's escape attempt. Once having freed them, he freed Markalis so that she could try to stop a deadly biological agent from being used against the Cardassian settlement on Ventani II. Unfortunately, his "freeing" of Limis, Darcen, and Roshanak was immediately followed by Darcen fatally stabbing Roshanak, much to his dismay.
Medical examination later confirmed observation from his former colleagues that Grimaud carried the experimental strain of the virus of which augments were immune. ("A Cause of Greater Worth")