One whom others naturally and sometimes inexplicably turn to with their problems. They "soak" up the "stains" of their issues, leaving the teller feeling better at having relieved themselves of bearing the burden of their problems alone. Usually an uninvolved party who is considered to have a more objective opinion of the situation.
It happened again! I was at a party talking to this girl and five minutes later she was crying on my shoulder telling me about her parents divorce!
I am such a trauma sponge, all my friends talk to me when they're sad.