a person who takes your problem, issue , or situation, and owns it as theirs.
This person turns it to somehow affecting them, and will broadcast it as widely as required in order to get the desired level of sympathy. A magpie will even sacrifice friendships to maintain her/his position.
person 1: have you heard about Jane and Sally losing their cat?
person 2: yeah, and Anna loved that cat but Jane and Sally didn't tell her, she has to find out from Bob.
person 1: Oh Anna is being such a magpie...
person 2: well, they should've told her!