The phrase "sad sack" refers to a person who is constantly gloomy or downcast, often due to their own misfortune or lack of motivation. Full definition
Stranger Than Fiction had the same sad sack character who eventually must come out of his shell to experience the joys in life as subtle as enjoying a warm cookie before his impending death. (fatmovieguy.com)
They're the sentimental favorites, and they just so happen to be a secret baseball sad sack. (sbnation.com)
I've seen Jemaine Clement in various supporting roles over the years, but never really latched on to him as a performer (I saw not one episode of his «Flight of the Conchords «-RRB-, but he makes for a very appealing sad sack of a leading man. (chrisreedfilm.com)