First of all, you need to know what the term foil in literature means. It is a character whose flaws are often exaggerated in order to highlight the virtues of the protagonist. In Ulysses, our protagonist is Ulysses/Odysseus, and his son Telemachus can be thought of as his foil. Telemachus fails to show bravery when Penelope's suitors start dishonoring his father because he doesn't stand up to them. So on one hand we have Telemachus' cowardice and Odysseus' bravery based on everything he did throughout the poem.
