Lieutenant Hugh Armitage was an aristocrat who moved to Cornwall where he grew close to Demelza Poldark.
Meeting Demelza Poldark
Hugh found out he was in bad health and would eventually lose eyesight within six months. He meets Demelza Poldark and falls in love with her. Feeling he has limited time left to enjoy life, he pursues her even though she's married. Demelza at the time was attempting to move on from Ross's own infidelity and was very drawn to Hugh.
Hugh made a romantic appeal to Demelza at the beach, telling her in six months he would be living in darkness and he wanted to know what it meant to love and be loved. Demelza was very taken by the proposal, and she slept with him. Demelza wanted to keep the truth from Ross, but being in love with her, Hugh offered her the chance to run away with him. Demelza denied the offer, leaving him heartbroken.
Lord Falmouth, Hugh's uncle had an ambition for Hugh to continue on a path towards running for parliament. Many months later, Hugh sent a poem to Demelza declaring he loved her, but she hid the poem from Ross. Unfortunately, Ross knew of the betrayal and became paranoid about Hugh and Demelza's connection.
But Hugh's health had continued to deteriorate and he began suffering from migraines. An attempt was made to save him with the means of leeches, but it never worked. On Hugh's death bed, Demelza spoke with him one final time, at the end of which Hugh passed away. Demelza was devastated by Hugh's death, and the impact he made on Ross and Demelza's marriage continued to be felt.