The son of Odysseus in Homer's Odyssey. See, e.g., "Telemachus," Wikipedia.
GoK, Vintage ed., p. 28, "your female Telemachus." Janet Beaton, like Telemachus, was eager to throw out the unwelcome intruders occupying the house and has rashly attacked them. In this analogy, perhaps Lymond is Odysseus, his true character concealed (though Telemachus was actually aware of his father's identity). Compare The Odyssey, Book XXI.